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We review recent advances in neuromyelitis optica, an idiopathic inflammatory demyelinating disease of the central nervous system predominantly affecting optic nerves and spinal cord. We concentrate on a recently identified serum antibody biomarker, neuromyelitis optica immunoglobulin G (NMO-IgG), which distinguishes neuromyelitis optica from multiple sclerosis.NMO-IgG is detected by indirect immunofluorescence. Its presence and specificity for neuromyelitis optica was confirmed in diverse populations. Seropositivity is now incorporated into new diagnostic criteria for neuromyelitis optica. Testing for this biomarker has suggested that the neuromyelitis optica spectrum is broader than previously recognized. Recently, the molecular target of NMO-IgG was identified as aquaporin-4. Immunopathologic studies suggest that loss of aquaporin-4 immunostaining is detectable in early lesions of neuromyelitis optica. A B-cell-specific monoclonal antibody, rituximab, may be an effective treatment even in patients not responding to other treatments.Clinical, radiologic, and immunologic features distinguish neuromyelitis optica from other severe cases of multiple sclerosis. NMO-IgG is the first specific marker for a central nervous system demyelinating disease. The discovery of aquaporin-4 as the putative target of NMO-IgG, and recent data suggesting that aquaporin-4-specific antibodies are pathogenic may enhance our understanding of idiopathic inflammatory demyelinating diseases and their treatment.