SMN2 and proximal spinal muscular atrophy: Baseline NfL values in both body fluids, CSF and serum, of the 18 SMA patients before the initiation of nusinersen treatment (first dose, month 0) were significantly higher within the group of patients with 2 SMN2 copies than in those with >2 SMN2 copies (95% CI; CSF [3855, 23541], serum [327, 1019], P < 0.001; Fig. 2A and B).