Also, in all cases, CSF was positive for AQP4-Ab in both samples, if samples were taken during relapse, or negative, if samples were taken during remission; the only exception was one AQP4-Ab seropositive sample that was negative for CSF AQP4-Ab during a relapse of ON but positive during a relapse of myelitis. Here, AQP4 is linked to myelitis.