SPP1 and primary progressive multiple sclerosis: Regarding the clinical forms, higher levels of OPN were detected in PPMS patients compared to RRMS in both serum and plasma samples (Figures 6B, C) and, even with high heterogeneity, SPMS showed higher plasma levels than RRMS (Figure 6E), suggesting that circulating OPN was elevated in both progressive MS forms, while RRMS showed higher OPN levels than SPMS in CSF (Figure 6D), as previously observed by Agah et al.