NEFL and amyotrophic lateral sclerosis: Neurofilament in cerebrospinal fluid (CSF) and serum has emerged as a much-needed candidate biomarker for the fatal adult neurodegenerative disorder amyotrophic lateral sclerosis (ALS).1 The neurofilament light chain protein (NfL) is the main structural constituent of the neuroaxonal cytoskeleton and mainly found in large, myelinated axons.