AQP4 and neuromyelitis optica: Longitudinally extensive transverse myelitis (LETM) stretching over three or more vertebral segments on MRI is commonly seen in neuromyelitis optica spectrum disorders (NMOSD).2 The presence of antibodies against aquaporin-4 (AQP4) is an important diagnostic biomarker of NMOSD, but no assay is 100% sensitive, and the test is unavailable in some regions.3 AQP4 antibody positivity is crucial because immunosuppressive therapy is effective in reducing further attacks in NMOSD.4