As there is an urgent need for ALS treatments, plasma neurofilament light chain (NfL), a secondary endpoint, was identified as a surrogate biomarker because it is a biologically plausible biomarker linked to neuroaxonal injury and neurodegeneration, it is associated with ALS disease progression and was reduced in tofersen-treated patients. This evidence concerns the gene NEFL and amyotrophic lateral sclerosis.