AQP4 and myeloid sarcoma: It was unclear, until as recently as 2004, whether NMOSD was a distinct illness or simply a more severe variant of ‘optico-spinal’ multiple sclerosis (MS), when the potential antigenic target, the aquaporin-4 water channel, was discovered, and the two disorders could be consistently differentiated using aquaporin-4 antibodies (AQP4-Abs) [118,119].