HLA-DRB1 and myeloid sarcoma: Genome-wide linkage studies have indicated several possible susceptibility loci, but the only locus to be identified across most studies is the major histocompatibility complex (MHC) on chromosome 6p21, were the HLA-DRB1*1501 allele is a well established genetic risk factor for MS.3, 4 This locus does not, however, account for the whole genetic component of MS, and multiple loci with smaller contributions to disease susceptibility are likely to exist.