The observation that natalizumab normalizes cerebrospinal fluid levels of the ectopic lymphoid organ chemokine CXCL13 in RRMS provides one potential mechanism natalizumab could be beneficial in SPMS; cerebrospinal fluid CXCL13 is elevated in SPMS [35]. This evidence concerns the gene CXCL13 and relapsing-remitting multiple sclerosis.