The Nf-L changes observed in serum and CSF following ETV:IDS treatment suggest that serum Nf-L may be used as a treatment-responsive biomarker of neurodegeneration, as has been demonstrated for other neurodegenerative disorders like Batten’s disease (20) and spinal muscular atrophy (21). This evidence concerns the gene NEFL and juvenile neuronal ceroid lipofuscinosis.