Although three different G-protein coupled receptors have been identified for arginine vasopressin, a significant physiological role has been recognized only for the V2 subtype that controls water homeostasis. Identification of the gene encoding the V2 vasopressin (or antidiuretic hormone) receptor enabled researchers to test the hypothesis that mutations of this gene were responsible for X-linked recessive nephrogenic diabetes insipidus. The affected patients are unable to concentrate their urine and as a consequence live in constant danger of dehydration that can cause death, particularly in infancy, or lead to severe hypernatremia that can impair their intellectual and physical development. The danger of severe dehydration diminishes in the adult patients, although they remain highly susceptible to this condition for the rest of their lives.