The only patient with MDS so far tested for serum AQP4-IgG, a highly specific marker of NMO, was negative for that marker [23] (a second patient was negative for CSF AQP4-IgG [49]; however, CSF is not the specimen of choice when it comes to testing for AQP4-IgG [50]). Here, AQP4 is linked to neuromyelitis optica.