Neurofilament light chain, the smallest subunit of the cytoskeletal protein neurofilaments, has been proposed as a clinically relevant biomarker of neuronal axonal injury and degeneration in several disorders, such as Charcot–Marie–Tooth disease, Metachromatic Leukodistrophy, or Hereditary Spastic Paraplegias [23,24,25]. The gene discussed is NEFL; the disease is hereditary spastic paraplegia.