NMO may be associated with antibodies selective for aquaporin-4 (AQP4), a water channel located within the optic nerves and spinal cord.AQP4 is distributed in the periventricular region, corpus callosum, magnocellular nuclei of the hypothalamus, and brain stem.Downbeat nystagmus (DBN) is associated with structural, metabolic, and autoimmune disorders, but is rarely reported in NMO.We treated a 33-year-old female with known AQP4-positive NMO who presented with new-onset DBN. The gene discussed is AQP4; the disease is autoimmune disease.