AQP4 and congenital myasthenic syndrome: Lennon and colleagues discovered a biomarker for NMO; NMO-IgG is an autoantibody initially detected in the serum of 73% of NMO but less than 5% of CMS patients [10], which binds to aquaporin-4 (AQP4) [11], the most abundant water channel in the CNS [12-14].