Axonal damage leads to NfL release into the extracellular space, and hence, NfL levels in the cerebrospinal fluid or in peripheral blood serve as a biomarker of axonal damage and neurodegeneration in a range of neurological disorders including multiple sclerosis [19,20], Alzheimer’s disease, frontotemporal dementia, Parkinson’s disease, Huntington’s disease and amyotrophic lateral sclerosis [21,22]. Here, NEFL is linked to Parkinson disease.