In humans, heterozygous mutations in the genes coding for the HNF1 homeobox A (HNF1A), hepatocyte nuclear factor -4 alpha (HNF4A) and HNF1 homeobox B (HNF1B) genes cause maturity-onset diabetes of the young subtypes HNF1A-MODY and HNF4A-MODY, and the renal cysts and diabetes (RCAD) syndrome respectively. This evidence concerns the gene HNF1A and type 2 diabetes mellitus.