There are over a million single nucleotide polymorphisms (SNPs) in the human genome. SNPs are specific sites within a human genome at which some individuals will have one nucleotide present while other individuals will have a different one. SNPs begin their existence as point mutations, and they eventually become established in a population. This substitution must occur in a significant proportion (more than 1 percent) of a large population for it to be called a SNP. Here is an example: In the DNA sequence TAGC, a SNP occurs when the G base changes to a C, and the sequence becomes TACC. When SNPs occur within a gene, the protein that results usually remains somewhat functional.
![SNPs: Frequently Occurring Genetic Variants](https://webarchive.library.unt.edu/eot2008/20081006123959im_/http://www.cancer.gov/images/Documents/a3a0ee76-1bc5-4490-aeaa-2248702c668f/cancer22.jpg)
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