Neurophysins are acidic proteins with a molecular weight of approximately 10,000 Da that are rich in cysteine, glycine, and proline residues . The protein is double domain with a polypeptide chain of 93-95 residues with 14 cysteine residues forming 7 disulfide bridges . Domain I contains a COOH terminal with a disulfide loop; domain II lacks this COOH terminal disulfide loop . Based on the resemblance of the disulfide loop present on vasopressin and oxytocin, it is suggested that the hormones form covalent linkages to this disulfide loop present on the COOH terminal of domain I.