HNF4A and type 2 diabetes mellitus: As a key member of the TCF family of transcription factors, TCF7L2 (transcription factor 7-like 2) is unique in that its gene is the most significant type 2 diabetes (T2DM) candidate gene identified to date (7), and recently attention has focused on the molecular and physiological role of TCF7L2 in the tissues relevant to T2DM pathogenesis—most notably the liver (8–10), brain (11), fat (12) and the pancreas (13–15).