The differential diagnosis between Parkinson disease (PD) and atypical parkinsonian disorders (APD), i.e., multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD), is often difficult due to the overlapping symptomatology, especially during the early stages of the disease course.1,2 Although there are not yet any established diagnostic methods that can reliably separate PD from APD, neurofilament light chain (NfL) protein in CSF is a promising marker for APD. The gene discussed is NEFL; the disease is Classical progressive supranuclear palsy.