question: When is the protein NFL a biomarker?
answer: Neurofilament light protein (NFL), may be released into the cerebrospinal fluid (CSF) during pathological processes in the central nervous system (CNS).
  Neurofilament light chain is a prognostic biomarker in neurological disorders such as amyotrophic lateral sclerosis, frontotemporal degeneration, axonal injury, late-onset cerebellar ataxia, multiple sclerosis and head trauma.