These antibodies bind to the water channel aquaporin 4 and are highly specific, occurring in 70%–90% of patients in former NMO series, yet are not found in patients suffering transverse myelitis or optic neuritis as manifestation of systemic autoimmune or rheumatic disease.2 The presence of AQP4-IgG provides a possible distinction of NMO from other autoimmune neurologic disorders. Here, AQP4 is linked to optic neuritis.