Vasoactive intestinal peptide (VIP; MIM 192320) and pituitary adenylate cyclase activating polypeptide (PACAP; MIM 102980) are homologous peptides that function as neurotransmitters and neuroendocrine hormones. While the receptors for VIP and PACAP share homology, they differ in their substrate specificities and expression patterns. See VIPR1 (MIM 192321) and ADCYAP1R1(MIM 102981).