Taken together, the data suggests that CSF, serum and plasma NfL is a sensitive, but non-specific marker of disease activity in the CNS and peripheral nervous system (PNS), with additional benefit of being able to measure disease activity and severity, as demonstrated in MS and HD, as well as treatment response, as shown in MS or spinal muscular atrophy (SMA) [58, 59]. The gene discussed is NEFL; the disease is proximal spinal muscular atrophy.