Arg-vasopressin is a neurohypophysial hormone found in most mammals, which corresponds to aa21-28 of vasopressin-neurophysin 2-copeptin (AVP-NPII). Vasopressin has a direct antidiuretic action on the kidney and also causes vasoconstriction of the peripheral vessels. Defects in AVP-NPII are the cause of autosomal dominant and recessive neurohypophyseal diabetes insipidus (ADNDI and ARNDI).H-Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2 (Cys1-Cys6) or H-CYFQNCPRG-NH2 (Cys1-Cys6)