<h4>Background</h4>The co-occurrence of myasthenia gravis (MG) and neuromyelitis optica spectrum disorder (NMOSD) is rare, and their temporal sequence and shared pathogenesis remain poorly understood.<h4>Methods</h4>We present the case of a 43-year-old woman with pre-existing acetylcholine receptor antibody (AChR-Ab) positive ocular MG who developed aquaporin-4 antibody (AQP4-Ab) positive NMOSD. The gene discussed is AQP4; the disease is myasthenia gravis.