The following glossary was modified from the glossary on the National Human Genome Research Institute's Web site, available at http://www.nhgri.nih.gov.

allele: One of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variation in inherited characteristics such as hair color or blood type. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one).

amino acid: One of 20 different kinds of small molecules that link together in long chains to form proteins. Amino acids are referred to as the "building blocks" of proteins.

autosomal dominant: Gene on one of the autosomes that, if present, will almost always produce a specific trait or disease. The chance of passing the gene (and therefore the disease) to children is 50-50 in each pregnancy.

autosome: Chromosome other than a sex chromosome. Humans have 22 pairs of autosomes.

base pair: Two bases that form a "rung of the DNA ladder." The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

birth defect: Defect present at birth, whether caused by mutant genes or by prenatal events that are not genetic.

BRCA1 and BRCA2: First breast cancer genes to be identified. Mutated forms of these genes are believed to be responsible for about one-half the cases of inherited breast cancer, especially those that occur in younger women, and also to increase a woman's risk for ovarian cancer. Both are tumor-suppressor genes.

cancer: Diseases in which abnormal cells divide and grow unchecked. Cancer can spread from its original site to other parts of the body and can be fatal if not treated adequately.

candidate gene: Gene, located in a chromosome region suspected of being involved in a disease, whose protein product suggests that it could be the disease gene in question.

CCR5: Mutation that confers immunity to infection by HIV. The mutation alters the structure of a receptor on the surface of macrophages such that HIV cannot enter the cell.

cDNA library: Collection of DNA sequences generated from mRNA sequences. This type of library contains only protein-coding DNA (genes) and does not include any noncoding DNA.

cell: Basic unit of any living organism. It is a small, watery compartment filled with chemicals and a complete copy of the organism's genome.

chromosome: One of the thread-like "packages" of genes and other DNA in the nucleus of a cell. Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and 2 sex chromosomes. Each parent contributes one chromosome to each pair, so children get one half of their chromosomes from their mothers and one half from their fathers.

cloning: Process of making copies of a specific piece of DNA, usually a gene. When geneticists speak of cloning, they do not mean the process of making genetically identical copies of an entire organism.