In 2004, Lennon and colleagues described a novel IgG serum reactivity present in around 60 to 80% of patients with NMO (termed NMO-IgG) [5,6], which was subsequently shown to target aquaporin-4 (AQP4), the most abundant water channel in the central nervous system (CNS) [7,8]. The gene discussed is AQP4; the disease is neuromyelitis optica.