Although more than 99% of human DNA sequences are the same across the population, variations in DNA sequence can have a major impact on how humans respond to disease, bacteria, viruses, toxins, drugs and other therapies. Single nucleotide polymorphisms (SNPs) are DNA sequence variations that occur when a single nucleotide (A, C, G or T) in the genome sequence is altered. For a variation to be considered a SNP, it must occur in at least 1% of the population. SNPs are evolutionarily stable - not changing much from generation to generation – which makes them perfect markers for population genetic studies [1] .

SNPs, which make up about 90% of all human genetic variation, occur every 100 to 300 bases along the 3-billion-base human genome. Two out of every three SNPs involve the replacement of cytosine (C) with thymine (T). SNPs can occur in both coding (gene) and non-coding regions of the genome (e.g., promoter sequences). Many SNPs have no effect on normal cell function but have been shown to correlate with disease predisposition or to influence drug activity [2].