Apelin is a neuropeptide expressed in the supraoptic and paraventricular nuclei that acts on specific receptors located on vasopressinergic neurons. The apelin gene to Xq25-q26.3 based on sequence similarity between the apelin sequence and a chromosome X PAC clone. This gene encodes a peptide that functions as an endogenous ligand for the G protein coupled receptor APJ. The encoded protein is synthesized as a prepropeptide that is processed into biologically active C-terminal fragments. The peptide fragments activate different tissue specific signaling pathways that regulate diverse biological functions including fluid homeostasis, cardiovascular function and insulin secretion. This protein also functions as a coreceptor for the human immunodeficiency virus 1.