The CXCR5 ligand CXCL13, the most potent B-cell chemoattractant, is detected in active MS lesions and in the CSF of patients with MS.7 CXCL13 was shown to be associated with disease exacerbations and unfavorable prognosis in MS and high CXCL13 levels predicted the conversion of clinical isolated syndrome (CIS) to MS.7 Here, CXCR5 is linked to myeloid sarcoma.