The 9 patients who were seropositive for AQP4 autoantibodies by cell-based IIFA while seronegative for NMO-IgG by tissue-based IIFA included patients with NMO (3), relapsing myelitis with LETM (3), a single attack of LETM (1), relapsing ON (1) and a single attack of ON (1); five of the 9 scored weakly positive (+) and the remaining 4 scored positive (++) on cell-based IIFA. The gene discussed is AQP4; the disease is neuromyelitis optica.