MTNR1B and pancreatic insulinoma: With regard to the recent identification of melatonin receptors in pancreatic β-cells [34]; [35] and melatonin's regulatory (i.e., inhibitory) effect on insulin secretion of INS1 insulinoma cells and rat pancreatic islets in vitro [36]–[38], it is conceivable that common genetic variation within MTNR1B affects β-cell function directly.