The remaining two seronegative patients fulfilled the 2015 IPND pathway for AQP4-IgG–negative/unknown NMOSD: each had ≥2 core clinical characteristics with dissemination in space-acute optic neuritis plus longitudinally extensive transverse myelitis (≥3 vertebral segments) on spinal MRI-no MS-typical brain MRI features, and alternative diagnoses excluded; MOG-IgG was negative where tested. Here, AQP4 is linked to myeloid sarcoma.