B‐cell receptor sequencing has shown that CSF‐infiltrating B‐cells exhibit evidence of somatic mutation69, 79, 80, 84, 85, 88 and CSR.79, 80, 83, 84 Indeed, ectopic lymphoid tissues have been found in the meninges of patients with MS and these exhibit germinal centre activity, and both IgM and class‐switched B‐cells have been found in the CSF. This evidence concerns the gene CD40LG and myeloid sarcoma.