Monoclonal antibody 4H1 recognizes Aquaporin-4. Aquaporin 4, also known as AQP4, is a protein which in humans is encoded by the AQP4 gene. AQP4 belongs to the aquaporin family of integral membrane proteins that conduct water through the cell membrane. AQP4 is constitutively expressed in the basolateral cell membrane of principal collecting duct cells in the kidney 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 is one of the most abundant molecules in the brain and is particularly prevalent in astrocytic membranes at the blood-brain and brain-liquor interfaces. While AQP4 has been implicated in a number of pathophysiological processes, its role in brain physiology has remained elusive. Only recently has evidence accumulated to suggest that AQP4 is involved in such diverse functions as regulation of extracellular space volume, potassium buffering, cerebrospinal fluid circulation, interstitial fluid resorption, waste clearance, neuroinflammation, osmosensation, cell migration, and Ca(2+) signaling. AQP4 is also required for normal function of the retina, inner ear, and olfactory system.Aquaporin-4 is the primary autoimmune target in neuromyelitis optica, in which AQP4-specific IgG antibodies target astrocytic AQP4, resulting in complement activation and inflammation.