AQP4 and neuromyelitis optica: NMOSD is distinguished from MS with the recognition of aquaporin-4 immunoglobulin G (AQP4-IgG), also known as NMO-IgG, which acts as an autoantibody against AQP4 that is a water channel in the brain parenchyma.11 As far as the authors of the present study are concerned, the association between smoking and NMO-IgG sero-positivity has not been investigated yet.