In this analysis, the C4 BS haplotype—which carries only a single C4 gene (a short form of C4B)—was the haplotype most associated with NMO-IgG+ risk (OR = 4.37, P = 7 × 10−13), followed by another single-gene haplotype which also carries only a single C4 gene (AL) (OR = 1.81, P = 0.049) (Fig. 3a). Here, C4A is linked to neuromyelitis optica.