SNPs occur about once every 1000 base pairs in the genome, making up the bulk of the 3 million variations found in the genome, and the frequency of a particular polymorphism tends to remain stable in the population.

Unlike the other, rarer kinds of variations, many SNPs occur in genes and in the surrounding regions of the genome that control their expression.

The effect of a single SNP on a gene may not be large - perhaps influencing the activity of the encoded protein in a subtle way - but even subtle effects can influence susceptibility to common diseases, such as heart disease or Alzheimer's disease.