In the subset of Pre-fALS participants with a SOD1 variant associated with rapid disease progression (e.g., p.Ala5Val [A5V; A4V]) and in whom clinically manifest ALS emerged (i.e., phenoconversion) during follow-up, increased levels of serum neurofilament, most notably neurofilament light chain (NfL), were observed 6–12 months prior to phenoconversion [26, 27]. Here, NEFL is linked to amyotrophic lateral sclerosis.