The nonapeptide vasopressin is synthetized as part of a longer common precursor polypeptide, together with its carrier protein neurophysin and a glycopeptide of unknown function. The gene for this common precursor has been isolated and sequenced and shown to comprise three exons encoding, respectively, the protein domains approximately corresponding to the N-terminal leader peptide and hormone, to most of the neurophysin, and to the glycopeptide. In the Brattleboro rat, the hereditary hypothalamic diabetes insipidus, characteristic of this strain, has been shown to be caused by a single nucleotide deletion in the vasopressin gene. This leads to the much reduced synthesis of a mutant vasopressin precursor, whose C terminus is quite unlike that in wild-type rats and which does not appear to be correctly processed in vivo.