SNCA and sleep disorder: Although a previous cross-sectional study in PD patients have revealed no association between CSF alpha-synuclein levels and RBD (46), prior investigations have revealed that CSF Prion Proteins (PrPs) are significantly elevated in PD patients with RBD compared to PD patients without sleep disorders (47), which might be suggestive of accelerated neuronal degeneration in PD-RBD patients and thus, introduce RBD as potentially the most appropriate clinical predictive marker of neuronal degeneration and disease progression in PD, as shown previously (48).