Serum NfL (sNfL) has shown potential for assisting the diagnosis and tracking the disease progression, for example, in multiple sclerosis,4 neurodegenerative diseases, such as Alzheimer’s disease5 and amyotrophic lateral sclerosis (ALS),6 epilepsy7 and traumatic brain injury.8 This serum-based NfL measurement paves the way for its use in clinical practice, including monitoring disease progression and treatment response and serving as an outcome measure in clinical trials. This evidence concerns the gene NEFL and multiple sclerosis.