Even though HNF-1A and HNF-4A are composed of different protein domains (Figure 1C) and exert specific gene regulatory functions (17), variants in the HNF1A and HNF4A genes lead to MODY forms of similar clinical phenotype (HNF1A-MODY and HNF4A-MODY, respectively) and are associated with neonatal hyperinsulinism (18–20). Here, HNF4A is linked to familial hyperinsulinism.