Apelin-13, an endogenous neuropeptide, is the ligand for the G-protein-coupled receptor APJ, an alternative coreceptor with CD4 for HIV-1 infection. Apelin-13 is also involved in the learning and memory process.Human: H-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe-OH or H-QRPRLSHKGPMPF-OH