The low proportion of follow-up EDSS data (25%) of the present PPMS cohort may have been a limiting factor, as correlations between EDSS and CSF biomarkers have been found in some studies (mainly of RRMS patients), for example between EDSS and CXCL-13 [31]. The gene discussed is CXCL13; the disease is relapsing-remitting multiple sclerosis.