NfL is a marker of degeneration of large myelinated axons.18 It is thus increased in CSF of several neurologic disorders, including acute conditions such as stroke19 and traumatic brain injury,20 as well as in chronic neurodegenerative disorders such as APD (PSP, MSA, and CBD),3, –, 7 amyotrophic lateral sclerosis,14,15,21 and frontotemporal dementia.22,23 CSF and blood NfL are not increased in PD, which could be due in part to less severe and widespread axonal degeneration in PD compared to APD. This evidence concerns the gene NEFL and amyotrophic lateral sclerosis.