HLA haplotype analyses of patients with NMO suggest a positive association with HLA-DRB1* 03:01 (DR17) [7, 8] [9], a gene that codes for a major histocompatibility class (MHC) II molecule that presents linear antigens to CD4+ T cells [10]. The gene discussed is HLA-DRB1; the disease is neuromyelitis optica.