HNF4A and diabetes mellitus: The central role of β cell in diabetes can be perceived by the fact that the majority of the identified single-gene mutations leading to diabetes, such as mutations in PAX 6 [8], HNF-1a, HNF-4a [9], GCK [10], PDX1 (Ipf1), etc. [11]), are involved with certain aspects of β cell biology including development, function, differentiation, proliferation, and apoptosis.