Familial central diabetes insipidus (FCDI) is a disorder of renal water conservation due to an inherited deficiency of the peptide hormonearginine vasopressin (AVP). Affected indi-viduals are normal at birth but become symptomatic with polyuria and polydipsia typically at 1-6 year of age due to progressive AVPdeficiency (1). Affected membersof FCDIfamilies have been found to be heterozygous for mutations in the AVPgene. The AVPgene is located on humanchromosome20 and consists of 3 exons (2). Exon1 encodes the signal peptide, AVP, and the aminoterminal region ofneurophysin II (NPII). Exon2accounts for the central region ofNPII, and exon 3 contains the carboxyterminal region of NPII and glycoprotein (3). Four different FCDImutations have been described that encode changes in the signal peptide