Detecting aquaporin-4 (AQP4) antibody is a key laboratory finding in the diagnosis of neuromyelitis optica (NMO) or its limited forms, known as NMO spectrum disorders (NMOSD).1,2 It is important to differentiate NMO from multiple sclerosis (MS), as drugs approved for MS (interferon-β, natalizumab, and fingolimod) are ineffective in NMO/NMOSD. AQP4 antibody seropositivity at onset of longitudinally extensive transverse myelitis (LETM), a cardinal feature of NMO, predicts relapses.3 Moreover, AQP4 antibody can damage astrocytes and cause NMO-like pathology.4 For these reasons, most neurologists now think that AQP4 antibody seropositivity is critically important in diagnosing NMO/NMOSD. The authors thank Prof. Yasuto Itoyama, National Center Hospital, NCNP, Tokyo, for suggestions.