Adult-onset, or type II diabetes mellitus (T2DM) has a complex genetic architecture, from hundreds of genes with low penetrance, common susceptibility variants (e.g., TCF7L2), to a set of more than ten genes that, when mutated, can cause a single-gene or Mendelian form of T2DM (e.g., GCK). Here, TCF7L2 is linked to type 2 diabetes mellitus.