Single Nucleotide Polymorphism.  A SNP is a variant in DNA where individuals will differ at a single nucleotide position. For example, one person may have a ‘C’ (cytosine) and another individual may have a ‘G’ (guanine) at a particular site along the DNA.  These variations are called ‘alleles’ for this particular position. Variants will usually be present in a frequency of at least > 1% in a population.  SNPs occur approximately every 300 base pairs which means there may be over 10 million SNPs in the human genome. Usually found between genes, they may have no significance or may be associated with metabolic processes or drug responses. If the SNPs occur close or within a gene, they may associated with disease.