AQP4 and transverse myelitis: We purified serum IgG from 24 serum samples from patients with NMO spectrum disorder (NMOSD), who were positive for anti‐AQP4 antibodies (longitudinally extensive transverse myelitis [LETM], n = 14; optic neuritis [ON], n = 6; other phenotype, n = 4) and nine healthy controls.