Welcome to the NCI Dictionary of Genetics Terms, which contains definitions for more than 100 terms related to genetics. These definitions were developed by the PDQ® Cancer Genetics Editorial Board to support the comprehensive, evidence-based, peer-reviewed PDQ genetics cancer information summaries.

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z 

A

affected
Individuals in a pedigree who exhibit the specific phenotype under study.

allele
One of two or more DNA sequences occurring at a particular gene locus. Typically one allele (“normal” DNA sequence) is common, and other alleles (mutations) are rare. (NCI Thesaurus)

allelic heterogeneity
Different mutations in the same gene that cause different phenotypic manifestations or severity of disease.

aneuploidy
The occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement, or any chromosome number that is not an exact multiple of the haploid number (which is 46). (NCI Thesaurus)

Ashkenazi Jews
One of two major ancestral groups of Jewish individuals, comprised of those whose ancestors lived in Eastern Europe (Germany, Poland, Russia). The other group is designated Sephardic Jews and includes those whose ancestors lived in North Africa, the Middle East, and Spain. Most Jews living in the United States are of Ashkenazi descent. Also called Eastern European Jews. (NCI Thesaurus)

assisted reproductive technology
ART. A term used to describe collectively a number of noncoital methods of conception that are used to treat infertility with donor or nondonor eggs and sperm including in vitro fertilization (IVF), gamete intrafallopian transfer (GIFT), and zygote intrafallopian transfer (ZIFT).

attributable risk
Proportion of a disease in exposed individuals that can be attributed to an exposure. In the context of genetic studies, the "exposure" is the frequency of a specific genetic variant.

autosomal
Refers to any of the chromosomes numbered 1-22 or the genes on chromosomes 1-22. This term excludes the sex-determining chromosomes, X and Y.

autosomal dominant
Autosomal dominant inheritance refers to genetic conditions that occur when a mutation is present in one copy of a given gene (i.e., the person is heterozygous).

autosomal recessive
Autosomal recessive inheritance refers to genetic conditions that occur only when mutations are present in both copies of a given gene (i.e., the person is homozygous for a mutation, or carries two different mutations of the same gene, a state referred to as compound heterozygosity).

B

band
When chromosomes at a particular stage in cell division are stained using one of several laboratory techniques, a specific pattern of light and dark stripes (bands) appears when the chromosomes are viewed through a microscope; the banding pattern assists in assigning each chromosome its particular number and evaluating its structure. (NCI Thesaurus)

base pair
Two nitrogen-containing bases pair together between double-stranded DNA; only specific combinations of these bases (e.g., adenine with thymine; guanine with cytosine) are possible, a fact which facilitates accurate DNA replication; when quantified (e.g., 8 base pairs, or bp), this term refers to the actual number of base pairs in a sequence of nucleotides.

C

cancer screening
Clinical testing designed to identify the presence of a specific cancer in an asymptomatic individual or population thought to be at risk of that specific cancer. The intent is to find cancers at the earliest possible stage in their development, in order to improve the chances for disease cure. (NCI Thesaurus)

carrier
In classical genetics, an individual who carries one deleterious allele for an autosomal recessive disorder. In clinical discussions, may refer to an individual who carries a deleterious allele that predisposes to disease.

carrier frequency
The proportion of individuals in a population who have a single copy of a specific recessive gene mutation; also sometimes applied to the prevalence of mutations in dominantly acting genes such as BRCA1 and BRCA2. Also called carrier rate.

carrier rate
The proportion of individuals in a population who have a single copy of a specific recessive gene mutation; also sometimes applied to the prevalence of mutations in dominantly acting genes such as BRCA1 and BRCA2. Also called carrier frequency.

chromosome
Discrete physical structures inside a cell nucleus that consist of proteins and DNA organized into genes. (NCI Thesaurus)

clone
An identical copy of a DNA sequence or entire gene; one or more cells derived from and identical to a single ancestor cell OR to isolate a gene or specific sequence of DNA. (NCI Thesaurus)

CNV
Refers to the genetic trait involving the number of copies of a particular gene present in the genome of an individual. Genetic variants, including insertions, deletions, and duplications of segments of DNA, are also collectively referred to as copy number variants. Copy number variants account for a significant proportion of the genetic variation between individuals. Also called copy number variant.

codon
In DNA or RNA, a sequence of 3 consecutive nucleotides that codes for a specific amino acid or signals the termination of gene translation (stop or termination codon). (NCI Thesaurus)

conformation-sensitive gel electrophoresis
CSGE. A type of mutation testing in which a segment of DNA is screened for mismatched pairing between normal and mutated base pairs. Also called CSGE.

congenital
A condition or trait present at birth, whether the result of a genetic or non-genetic factors.

consanguinity
Genetic relatedness between individuals who are descendants of at least one common ancestor.

consultand
An individual who presents for genetic counseling. Also called counselee.

copy number variant
Refers to the genetic trait involving the number of copies of a particular gene present in the genome of an individual. Genetic variants, including insertions, deletions, and duplications of segments of DNA, are also collectively referred to as copy number variants. Copy number variants account for a significant proportion of the genetic variation between individuals. Also called CNV.

cosegregation
The transmission, together, of 2 or more genes on the same chromosome, as a result of their being in very close physical proximity to one another (i.e., linked).

counselee
An individual who presents for genetic counseling. Also called consultand.

CSGE
Conformation-sensitive gel electrophoresis. A type of mutation testing in which a segment of DNA is screened for mismatched pairing between normal and mutated base pairs. Also called conformation-sensitive gel electrophoresis.

cytogenetics
The study of the structure, function, and abnormalities of human chromosomes. (NCI Thesaurus)

D

de novo mutation
An alteration in a gene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents, or a mutation that arises in the fertilized egg itself during early embryogenesis. Also called new mutation. (NCI Thesaurus)

deletion
Absence of a segment of DNA; may be as small as a single base or as large as a whole chromosome. (NCI Thesaurus)

deleterious mutation
A mutation that is documented to be associated with risk of disease.

deoxyribonucleic acid
DNA. The molecular basis of heredity; encodes the genetic information responsible for the development and function of an organism and allows for transmission of that genetic information from one generation to the next. The DNA molecule is structured as a double-stranded helix held together by weak hydrogen bonds between purine-pyrimidine nucleotide base pairs: adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (C). Also called DNA. (NCI Thesaurus)

dirty necrosis
Presence of necrotic cellular debris within the lumen of the neoplastic glands in the colorectal mucosa.

disease-causing mutation
A gene alteration that causes or predisposes an individual to a specific disease.

DNA
Deoxyribonucleic acid. The molecular basis of heredity; encodes the genetic information responsible for the development and function of an organism and allows for transmission of that genetic information from one generation to the next. The DNA molecule is structured as a double-stranded helix held together by weak hydrogen bonds between purine-pyrimidine nucleotide base pairs: adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (C). Also called deoxyribonucleic acid. (NCI Thesaurus)

domain
A specific physical region or amino acid sequence in a protein which is associated with a particular function or corresponding segment of DNA.

E

Eastern European Jews
One of two major ancenstral groups of Jewish individuals, comprised of those whose ancestors lived in Eastern Europe (Germany, Poland, Russia). The other group is designated Sephardic Jews and includes those whose ancestors lived in North Africa, the Middle East, and Spain. Most Jews living in the United States are of Eastern European descent. Also called Ashkenazi Jews. (NCI Thesaurus)

exon
Coding sequence of DNA present in mature messenger RNA. Most genes have multiple exons. (NCI Thesaurus)

F

false-positive result
A test result that indicates an individual is affected and/or has a certain gene mutation when he or she is actually unaffected and/or does not have the mutation; i.e., a positive test result in a truly unaffected or mutation-negative individual. (NCI Thesaurus)

familial
A phenotype or trait that occurs with greater frequency in a given family than in the general population; familial traits may have a genetic and/or nongenetic etiology.

family history
The genetic relationships within a family combined with the medical history of individual family members. When represented in diagram form using standardized symbols and terminology, it is usually referred to as a pedigree or family tree. (NCI Thesaurus)

FDR
First-degree relative. The parents, siblings, or children of an individual. Also called first-degree relative. (NCI Thesaurus)

first-degree relative
FDR. The parents, siblings, or children of an individual. Also called FDR. (NCI Thesaurus)

FISH
Fluorescence in situ hybridization. A technique used to identify the presence of specific chromosomes or chromosomal regions through hybridization (attachment) of fluorescently-labeled DNA probes to denatured chromosomal DNA. Examination through a microscope under fluorescent lighting detects the presence of the colored hybridized signal (and hence presence of the chromosome material) or absence of the hybridized signal (and hence absence of the chromosome material). Also called fluorescence in situ hybridization. (NCI Thesaurus)

fluorescence in situ hybridization
FISH. A technique used to identify the presence of specific chromosomes or chromosomal regions through hybridization (attachment) of fluorescently-labeled DNA probes to denatured chromosomal DNA. Examination through a microscope under fluorescent lighting detects the presence of the colored hybridized signal (and hence presence of the chromosome material) or absence of the hybridized signal (and hence absence of the chromosome material). Also called FISH. (NCI Thesaurus)

founder effect
A gene mutation observed with high frequency in a population founded by a small ancestral group that was once geographically or culturally isolated, in which one or more of the founders was a carrier of the mutant gene.

frameshift mutation
An insertion or deletion involving a number of base pairs that is not a multiple of three, which consequently disrupts the triplet reading frame of a DNA sequence. Such mutations usually lead to the creation of a premature termination (stop) codon, and result in a truncated (shorter-than-normal) protein product. (NCI Thesaurus)

G

gene
The basic unit of heredity that occupies a specific location on a chromosome. Each consists of nucleotides arranged in a linear manner. Most genes code for a specific protein or segment of protein leading to a particular characteristic or function. (NCI Thesaurus)

genetic counseling
A communication process that seeks to assist affected or at-risk individuals and families in understanding the natural history, disease risks, and mode of transmission of a genetic disorder; to facilitate informed consent for genetic testing when appropriate; to discuss options for risk management and family planning; and to provide for or refer individuals for psychosocial support as needed. The National Society of Genetic Counselors Task Force has also defined the term genetic counseling. (NCI Thesaurus)

genetic heterogeneity
The production of the same or similar phenotypes (observed biochemical, physiological, and morphological characteristics of a person determined by his/her genotype) by different genetic mechanisms. There are two types: (1) allelic heterogeneity – when different alleles at a locus can produce variable expression of a condition; and (2) locus heterogeneity – the term used to describe disease in which mutations at different loci can produce the same disease phenotype.

genetic marker
An identifiable segment of DNA (e.g., Single Nucleotide Polymorphism [SNP], Restriction Fragment Length Polymorphism [RFLP], Variable Number of Tandem Repeats [VNTR], microsatellite) with enough variation between individuals that its inheritance and co-inheritance with alleles of a given gene can be traced; used in linkage analysis. (NCI Thesaurus)

genetic predisposition
Increased likelihood or chance of developing a particular disease due to the presence of one or more gene mutations and/or a family history that indicates an increased risk of the disease. Also called genetic susceptibility. (NCI Thesaurus)

genetic screening
Genetic testing designed to identify individuals in a given population who are at higher risk of having or developing a particular disorder, or carrying a gene for a particular disorder.

genetic susceptibility
Increased likelihood or chance of developing a particular disease due to the presence of one or more gene mutations and/or a family history that indicates an increased risk of the disease. Also called genetic predisposition. (NCI Thesaurus)

genomic imprinting
The process by which one chromosome of a pair is chemically modified, depending on whether the chromosome comes from the father or the mother. These modifications lead to differential expression of a gene or genes on a maternally derived chromosome versus a paternally derived chromosome. (NCI Thesaurus)

germline
The cells from which eggs or sperm (i.e., gametes) are derived.

H

haplotype
A set of closely linked genetic markers present on one chromosome which tend to be inherited together.

heterozygous genotype
Occurs when the two alleles at a particular gene locus are different. A heterozygous genotype may include one normal allele and one mutation, or two different mutations. The latter is called a compound heterozygote.

homozygous genotype
Occurs when both alleles at a particular gene locus are the same. A person may be homozygous for the normal allele or for a mutation.

I

inconclusive
A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called uninformative and indeterminate.

indeterminate
A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of his or her personal and family history. Also called uninformative and inconclusive.

index case
A clinically affected individual through whom attention is first drawn to a genetic disorder in a family.

informative
In genetic testing, a test result that reveals definitively the presence or absence of the germline genetic alteration associated with the hereditary disorder being assessed. In linkage analysis, the ability to distinguish between maternally inherited and paternally inherited DNA markers (polymorphisms) within or near a given gene of interest.

informed consent
A process of information exchange between a clinician and an individual or their legal proxy designed to facilitate autonomous, informed decision making. The informed consent process for genetic testing should include an explanation of the medical and psychosocial risks, benefits, limitations, and potential implications of genetic analysis, a discussion of privacy, confidentiality, the documentation and handling of genetic test results, as well as options for managing the hereditary disease risk. (NCI Thesaurus)

inherited cancer syndrome
Describes the clinical manifestations associated with a mutation conferring cancer susceptibility.

J

No entries.

K

kindred
An extended family.

L

late or variable onset
The state in which a genetic trait is expressed later in life or is expressed at no fixed time in a life history.

linkage
The tendency for genes or segments of DNA closely positioned along a chromosome to segregate together at meiosis, and therefore be inherited together. (NCI Thesaurus)

linkage analysis
A gene-hunting technique that traces patterns of disease in high-risk families. It attempts to locate a disease-causing gene by identifying genetic markers of known chromosomal location that are co-inherited with the trait of interest.

locus
The physical site or location of a specific gene on a chromosome. (NCI Thesaurus)

locus heterogeneity
The situation in which mutations in genes at different chromosomal loci result in the same phenotype.

LOD score
Logarithm of the odd score. A statistical estimate of whether two genetic loci are physically near enough to each other (or "linked") on a particular chromosome that they are likely to be inherited together. A LOD score of three or more is generally considered statistically significant evidence of linkage. Also called logarithm of the odd score. (NCI Thesaurus)

logarithm of the odd score
LOD score. A statistical estimate of whether two genetic loci are physically near enough to each other (or "linked") on a particular chromosome that they are likely to be inherited together. A LOD score of three or more is generally considered statistically significant evidence of linkage. Also called LOD score. (NCI Thesaurus)

LOH
Loss of heterozygosity. If there is one normal and one abnormal allele at a particular locus, as might be seen in an inherited autosomal dominant cancer susceptibility disorder, loss of the normal allele produces a locus with no normal function. When the loss of heterozygosity involves the normal allele, it creates a cell that is more likely to show malignant growth if the altered gene is a tumor suppressor gene. Also called loss of heterozygosity. (NCI Thesaurus)

loss of heterozygosity
LOH. If there is one normal and one abnormal allele at a particular locus, as might be seen in an inherited autosomal dominant cancer susceptibility disorder, loss of the normal allele produces a locus with no normal function. When the loss of heterozygosity involves the normal allele, it creates a cell that is more likely to show malignant growth if the altered gene is a tumor suppressor gene. Also called LOH. (NCI Thesaurus)

M

microsatellite
Repetitive segments of DNA scattered throughout the genome in noncoding regions between genes or within genes (introns). They are often used as markers for linkage analysis because of their naturally occurring high variability in repeat number between individuals. These regions are inherently genetically unstable and susceptible to mutations. (NCI Thesaurus)

microsatellite instability
MSI. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called MSI.

missense mutation
A single base pair substitution that alters the genetic code in a way that produces an amino acid that is different from the normal amino acid at that position. (NCI Thesaurus)

mode of inheritance
The manner in which a genetic trait or disorder is passed from one generation to the next. Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, multifactorial, and mitochondrial inheritance are examples. Each mode of inheritance results in a characteristic pattern of affected and unaffected family members. (NCI Thesaurus)

mosaicism
The occurrence of 2 or more cell lines with different genetic or chromosomal make-up, within a single individual or tissue.

MSI
Microsatellite instability. A characteristic of cells that contain an abnormality in DNA mismatch repair (see microsatellite). For example, the presence of MSI in colorectal tumor tissue may be used as a marker for germline mutations in one of the DNA mismatch repair genes associated with HNPCC. MSI can also occur sporadically, and in these cases is related to gene hypermethylation. This is an issue in the differential diagnosis of HNPCC. Also called microsatellite instability.

mutation
A change in the usual DNA sequence at a particular gene locus. Mutations (including polymorphisms) can be harmful, beneficial, or neutral in their effect on cell function. (NCI Thesaurus)

mutation analysis
Germline genetic testing method targeted to detect a specific mutation (such as a deleterious MSH2 mutation previously identified in a family), panel of mutations (such as the 3 BRCA mutations comprising the founder mutation panel for individuals of Ashkenazi Jewish ancestry) or type of mutation (such as a large deletions or insertions in the BRCA1 gene). This type of testing is distinct from complete gene sequencing or mutation scanning. The latter are designed to detect most mutations in the region being tested. Current usage also applies this term to any genetic test. (NCI Thesaurus)

N

negative predictive value
NPV. The likelihood that an individual with a negative test result is truly unaffected and/or does not have the particular gene mutation in question. Also called NPV. (NCI Thesaurus)

new mutation
An alteration in a gene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents, or a mutation that arises in the fertilized egg itself during early embryogenesis. Also called de novo mutation. (NCI Thesaurus)

noncarrier
An individual who does not carry a mutation previously identified in his or her family.

nonpenetrance
The state in which a genetic trait, although present in the appropriate genotype, fails to manifest itself in the phenotype (e.g., a woman with a BRCA1 mutation who lives to be elderly and never develops breast or ovarian cancer).

novel mutation
A newly discovered, distinct gene alteration; NOT the same as new or de novo mutation.

NPV
Negative predictive value. The likelihood that an individual with a negative test result is truly unaffected and/or does not have the particular gene mutation in question. Also called negative predictive value. (NCI Thesaurus)

nucleotide
A molecule consisting of a nitrogen-containing base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate group, and a sugar (deoxyribose in DNA; ribose in RNA). DNA and RNA are polymers comprised of many nucleotides, strung together like beads in a necklace. (NCI Thesaurus)

null allele
A mutation that results in either no gene product or the absence of function at the phenotypic level.

O

No entries.

P

PCR
Polymerase chain reaction. A procedure that produces millions of copies of a short segment of DNA through repeated cycles of: (1) denaturation, (2) annealing, and (3) elongation. PCR is a very common procedure in molecular genetic testing and may be used to generate a sufficient quantity of DNA to perform a test (e.g., allele-specific amplification, trinucleotide repeat quantification). Also called polymerase chain reaction. (NCI Thesaurus)

pedigree
A graphic illustration of family history. (NCI Thesaurus)

penetrance
A characteristic of a genotype; it refers to the likelihood that a clinical condition will occur when a particular genotype is present. (NCI Thesaurus)

phenocopy
A phenotypic trait or disease that resembles the trait expressed by a particular genotype, but in an individual who is not a carrier of that genotype. For example, breast cancer in a hereditary breast/ovarian cancer syndrome family member who does not carry the family’s BRCA1 or BRCA2 mutation would be considered a phenocopy. Such an individual does not have the family’s cancer-related mutation and therefore, they do not have the associated cancer risk from that specific mutation.

phenotype
The observable characteristics in an individual resulting from the expression of genes; the clinical presentation of an individual with a particular genotype. (NCI Thesaurus)

point mutation
An alteration in a DNA sequence caused by the substitution of a single nucleotide for another nucleotide.

polymerase chain reaction
PCR. A procedure that produces millions of copies of a short segment of DNA through repeated cycles of: (1) denaturation, (2) annealing, and (3) elongation. PCR is a very common procedure in molecular genetic testing and may be used to generate a sufficient quantity of DNA to perform a test (e.g., allele-specific amplification, trinucleotide repeat quantification). Also called PCR. (NCI Thesaurus)

polymorphism
A common mutation. “Common” is typically defined as an allele frequency of at least 1%. All genes occur in pairs, except when x and y chromosomes are paired in males; thus a polymorphism with an allele frequency of 1% would be found in about 2% of the population, with most carriers having one copy of the polymorphism and one copy of the normal allele. (NCI Thesaurus)

population risk
The proportion of individuals in the general population who are affected with a particular disorder or who carry a certain gene; often discussed in the genetic counseling process as a comparison to the patient’s personal risk given his or her family history or other circumstances.

positive predictive value
PPV. The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called PPV. (NCI Thesaurus)

PPV
Positive predictive value. The likelihood that an individual with a positive test result truly has the particular gene and/or disease in question. Also called positive predictive value. (NCI Thesaurus)

predisposing mutation
A germline mutation that increases an individual’s susceptibility or predisposition to a certain disease or disorder. When such a mutation is inherited, development of symptoms is more likely, but not certain. Also called susceptibility gene. (NCI Thesaurus)

presymptomatic testing
Genetic analysis of an asymptomatic or unaffected individual who is at risk of a specific genetic disorder.

proband
The individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus, and in females it is called a proposita.

proposita
The female individual through whom a family with a genetic disorder is ascertained. In males this is called a propositus.

propositus
The male individual through whom a family with a genetic disorder is ascertained. In females this is called a proposita.

Q

No entries.

R

recurrence risk
In genetics, the likelihood that a hereditary trait or disorder present in one family member will occur again in other family members. This is distinguished from recurrence risk for cancer, which is the chance that a cancer that has been treated will recur.

risk assessment
The quantitative or qualitative assessment of an individual’s risk of carrying a certain gene mutation, or developing a particular disorder, or of having a child with a certain disorder; sometimes done by using mathematical or statistical models incorporating such factors as personal health history, family medical history and ethnic background.

S

SDR
Second-degree relative. The aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings of an individual. Also called second-degree relative. (NCI Thesaurus)

second-degree relative
SDR. The aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings of an individual. Also called SDR. (NCI Thesaurus)

sensitivity
The frequency with which a test yields a true positive result among individuals who actually have the disease or the gene mutation in question. A test with high sensitivity has a low false-negative rate and thus does a good job of correctly identifying affected individuals.

single nucleotide polymorphism
SNP. DNA sequence variations that occur when a single nucleotide (adenine, thymine, cytosine, or guanine) in the genome sequence is altered; usually present in at least 1% of the population; pronounced "snip". Also called SNP. (NCI Thesaurus)

single-stranded conformational polymorphism
SSCP. A type of mutation scanning; the identification of abnormally-migrating single-stranded DNA segments on gel electrophoresis. Also called SSCP. (NCI Thesaurus)

SNP
Single nucleotide polymorphism. DNA sequence variations that occur when a single nucleotide (adenine, thymine, cytosine, or guanine) in the genome sequence is altered; usually present in at least 1% of the population; pronounced "snip". Also called single nucleotide polymorphism. (NCI Thesaurus)

Southern blot
Electrophoresis-based technique used in genetic testing to detect large deletions in DNA that can be missed by PCR-based genetic testing methods. (NCI Thesaurus)

specificity
The frequency with which a test yields a true negative result among individuals who do not have the disease or the gene mutation in question. A test with high specificity has a low false-positive rate and thus does a good job of correctly classifying unaffected individuals.

splicing
The process by which introns, the noncoding regions of genes, are excised out of the primary messenger RNA transcript, and the exons (i.e., coding regions) are joined together to generate mature messenger RNA. The latter serves as the template for synthesis of a specific protein. (NCI Thesaurus)

sporadic cancer
This term has two meanings. It is sometimes used to differentiate cancers occurring in people who do not have a germline mutation that confers increased susceptibility to cancer from cancers occurring in people who are known to carry a mutation. Cancer developing in people who do not carry a high-risk mutation is referred to as sporadic cancer. The distinction is not absolute, because genetic background may influence the likelihood of cancer even in the absence of a specific predisposing mutation. Alternatively, sporadic is also sometimes used to describe cancer occurring in individuals without a family history of cancer.

SSCP
Single-stranded conformational polymorphism. A type of mutation scanning; the identification of abnormally-migrating single-stranded DNA segments on gel electrophoresis. Also called single-stranded conformational polymorphism. (NCI Thesaurus)

susceptibility gene
A germline mutation that increases an individual’s susceptibility or predisposition to a certain disease or disorder. When such a mutation is inherited, development of symptoms is more likely, but not certain. Also called predisposing mutation. (NCI Thesaurus)

T

transcription
The process of synthesizing messenger RNA (mRNA) from DNA. (NCI Thesaurus)

translation
The process of synthesizing an amino acid sequence (protein product) from the messenger RNA code. (NCI Thesaurus)

trinucleotide repeat
Sequences of 3 nucleotides repeated in tandem on the same contiguous section of chromosome. A certain amount of normal (polymorphic) variation in repeat number with no clinical significance commonly occurs between individuals; however, repeat numbers over a certain threshold can, in some cases, lead to adverse effects on the function of the gene, resulting in genetic disease.

trisomy
The presence of an extra chromosome, resulting in a total of three copies of that chromosome instead of the normal 2 copies (e.g., trisomy 21, or Down syndrome). (NCI Thesaurus)

U

unaffected
An individual who does not manifest symptoms of a condition or disease occurring in his or her family.

unclassified variant
A variation in a genetic sequence whose association with disease risk is unknown. Also called variant of unknown significance, variant of uncertain significance, and VUS.

uninformative
A negative test result in an individual where a clearly deleterious mutation has not been found in any family members. The genetic risk status of such an individual must be interpreted in the context of their personal and family history. Also called indeterminate and inconclusive.

V

variable expression
Variation in the manner in which a trait is manifested. When there is variable expressivity, the trait may vary in clinical expression from mild to severe. For example, the condition neurofibromatosis type 1 may be mild, presenting with café-au-lait spots only, or may be severe, presenting with neurofibromas and brain tumors.

variant of uncertain significance
VUS. A variation in a genetic sequence whose association with disease risk is unknown. Also called variant of unknown significance, unclassified variant, and VUS.

variant of unknown significance
VUS. A variation in a genetic sequence whose association with disease risk is unknown. Also called variant of uncertain significance, unclassified variant, and VUS.

VUS
Variant of uncertain significance. A variation in a genetic sequence whose association with disease risk is unknown. Also called variant of uncertain significance, variant of unknown significance, and unclassified variant.

W

No entries.

X

X-linked recessive
X-linked recessive inheritance refers to genetic conditions associated with mutations in genes on the X chromosome. A male carrying such a mutation will be affected, because he carries only one X chromosome. A female carrying a mutation in one gene, with a normal gene on the other X chromosome, will be unaffected.

Y

No entries.

Z

No entries.

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