The mechanisms by which the HLA-DRB1*15 haplotype affects MS susceptibility remain to be precisely defined but may include HLA-DRB1*15 hypomethylation causing changes in HLA-DRB1*15 expression on antigen presenting cells (APC) (92) and presentation of self peptides by APC that activate CD4 T cells recognizing antigens expressed in the CNS (72, 93, 94). The gene discussed is CD4; the disease is myeloid sarcoma.