The MHC makes the single largest genetic contribution to MS susceptibility in whites of European ancestry, on its own explaining ~20% of the heritability estimated from genotyped SNPs [1] and was first identified as a determinant of MS risk in the 1970s by utilizing lymphocyte cultures [2] and lymphocytotoxic antisera reactions [3]. The gene discussed is HLA-C; the disease is myeloid sarcoma.