Notably, the tremendous clinical importance of such “migratory” B cells is strongly underlined by our finding in a restricted number of patients, that CD27+ isotype switched B cell phenotypes change compartments, i.e. disappear in blood and accumulate intrathecally, when MS activity recurs after withdrawal from NAT therapy, thus qualifying this subset as an attractive biomarker of upcoming MS reactivation. The gene discussed is BRD2; the disease is myeloid sarcoma.