Single nucleotide polymorphisms (SNPs)
are the most frequent type of genetic variation among people. Each SNP represents a difference in a nucleotide, which is a single DNA building block. For example, an SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA. Just as changes in the letters of a word can alter its meaning, these genetic changes can alter the instructions given by the DNA in cells.