HNF1A and type 2 diabetes mellitus: For example, rare highly penetrant sequence variants in several genes (HNF4A, GCK, TCF1/HNF1A, TCF2/HNF1B) invariably cause a monogenic disorder known as maturity-onset diabetes of the young (MODY), while common less penetrant variants in the same genes are risk factors for multifactorial type 2 diabetes.