Single Nucleotide Polymorphism. Common, but minute, variations that occur in human DNA at a frequency of 1 every 1,000 bases. An SNP is a single base-pair site within the genome at which more than one of the four possible base pairs is commonly found in natural populations. Over 10 million SNP sites have been identified and mapped on the sequence of the genome, providing the densest possible map of genetic differences. SNP is pronounced “snip”.