NEFL and amyotrophic lateral sclerosis: When the axons of the central nervous system are damaged, NfL is released into the extracellular space, increasing NfL levels not only in the cerebrospinal fluid (CSF) but also in the peripheral blood.[1] This suggests that serum NfL may act as a biomarker of neuroaxonal damage in a variety of neurological disorders, including neurodegenerative diseases, multiple sclerosis, amyotrophic lateral sclerosis, and traumatic brain injury.[2–6]