On the one hand, humanised mice carrying the strongest MS risk allele HLA‐DRB1*15: 01 were shown by one study to have increased EBV load, defective CD4+ T‐cell control of EBV‐transformed B‐cell lines and T cells with reactivity for myelin basic protein (MBP) compared with animals carrying an HLA allele not associated with MS.63 Here, CD4 is linked to myeloid sarcoma.