While myelitis is typically longitudinally extensive—extending over three or more complete vertebral segments—in both AQP4-IgG-positive and AQP4-IgG-negative NMOSD (and MOG-EM/MOGAD), as mentioned above, long lesions are extremely rare in MS and, if present, usually result from the coalescence of adjacent lesions. This evidence concerns the gene AQP4 and myeloid sarcoma.