ADCYAP1 and neuroendocrine neoplasm: Vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase activating polypeptide (PACAP), and their receptors PAC1, VPAC1 and PAC2 have emerged as important factors in islet cell function (insulin secretion) and growth and differentiation of neuroendocrine tumors including insulinomas.