Kang H, et al. - Here, the authors evaluate the frequency of AQP4 antibody (AQP4-Ab) in Chinese patients with optic neuritis (ON) and examine the prognostic implications of AQP4-Ab seropositivity in such patients. In China, acute optic neuritis has a higher incidence of anti-aquaporin-4 antibody than in Western countries. For discriminating neuromyelitis optica (NMO)-ON from other types of optic neuritis during the early stage of the disease course, AQP4-Ab is a sensitive and specific biomarker.