Maturity-onset diabetes of the young type 3 (MODY 3) is a consequence of heterozygous germline mutations in HNF1A, and a subtype of hepatocellular adenoma (HCA) is caused by biallelic somatic HNF1A mutations; rare HCA may be related to MODY 3. The gene discussed is HNF1A; the disease is hepatocellular adenoma.