However, although the hallmark of MS pathology is the presence of oligoclonal bands in CSF, making antibody‐secreting B cells the obvious suspect, the GWAS enrichment studies all point to risk being mediated by gene regulation in CD4+ T cells43 a view reinforced by the success of α4 integrin blockade by natalizumab, which prevents T cells from crossing the blood–brain barrier and forming new lesions. This evidence concerns the gene CD4 and myeloid sarcoma.