Several lines of evidence implicate a major role for complement activation in NMO, including human pathology showing deposition of activated complement [16, 18, 26], rodent models showing complement-dependent NMO pathology following passive transfer of AQP4-IgG [1, 28, 37], and an open-label clinical trial of the C5 convertase inhibitor eculizumab showing efficacy in NMO [21]. Here, AQP4 is linked to neuromyelitis optica.