As expected, lower than normal total C4 (CN = 2 or CN = 3) was associated with higher risk for NMO-IgG+ (OR = 6.99, P = 7.20 × 10−6; OR = 5.10, P = 8.47 × 10−15 for CN = 2 and CN = 3 respectively compared with CN = 4). The gene discussed is C4A; the disease is neuromyelitis optica.