NEFL and Parkinson disease: Combination of the ratio o-α-syn/total α-syn (t-α-syn) and the age of patients, CSF t-α-syn and Aβ42, t-tau and p-tau, and β-glucocerebrosidase activity can differentiate PD patients from healthy subjects [98,99], while CSF p-α-syn/t-α-syn, CSF Aβ42, p-tau, and, in particular, CSF NfL levels are associated with cognitive performance in early-stage PD patients.