Aquaporin-2 (AQP-2) forms a water specific channel that provides the plasma membranes of renal collecting duct with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient. It is expressed in the renal collecting tubules. Defects in AQP-2 are a cause of autosomal recessive nephrogenic diabetes insipidus (ANDI), also known as diabetes insipidus nephrogenic type 2. It is characterized by excessive water drinking (polydypsia) and urine excretion (polyuria). AQP-2 belongs to the MIP/aquaporin (TC1.A.8) family. Ser-256 phosphorylation is necessary and sufficient for expression at the apical membrane.