NEFL and Parkinson disease: NfL levels have been investigated in a number of neurodegenerative disorders, including multiple sclerosis (MS) (Disanto et al., 2017[24]), Alzheimer's disease (AD) (Mattsson et al., 2017[68]), atypical Parkinsonian disorder (APD), Parkinson's disease (PD) (Hansson et al., 2017[45]), amyotrophic lateral sclerosis (ALS) (Lu et al., 2015[63]), frontotemporal dementia (FTD) (Rohrer et al., 2016[89]), and Huntington's disease (HD) (Constantinescu et al., 2009[19]; Byrne et al., 2017[14]).