Aquaporin-4 (AQP4), is a member of the aquaporin family of intrinsic membrane proteins that function as water-selective channels in the plasma membranes of many cells. The encoded protein is the predominant aquaporin found in brain. Aquaporin-4 is located in many tissues, including the basolateral membrane of the renal cortical and medullary principal cell, as well as in the brain.Aquaporin-4 is the first specific molecule to be defined as a target for the autoimmune response in any form of multiple sclerosis. Aquaporin 4 is found in the basolateral cell membrane of principal collecting duct cells and provide a pathway for water to exit these cells.AQP4 is also expressed in astrocytes and is upregulated by direct insult to the central nervous system.Aquaporin-4 has been proposed as the primary autoimmune target in Devic's disease.