Population genetics: SNPs that come in threes

Single-nucleotide polymorphisms (SNPs) are the bread and butter of many studies of sequence variation, and so understanding how they vary is useful to studies of genome evolution and disease susceptibility. Most human SNPs are biallelic — that is, two allelic variants are segregating in the population — but a paper now shows that there are twice as many triallelic SNPs as expected, and puts forward a mutational mechanism by which they might arise.