Plasma and serum NfL have been reported to have a diagnostic sensitivity ranging from 76–100% and a specificity of 75–92%.14–17 However, studies have typically compared ALS patient samples either with those from predominantly healthy controls or patients retrospectively labelled as having other neurological conditions, many of which would be readily distinguished from ALS on clinical grounds alone. This evidence concerns the gene NEFL and amyotrophic lateral sclerosis.