SNCA and Parkinson disease: Plasma pS129 SNCA levels were found to be higher in PD samples than in samples from control individuals (Foulds et al., 2011, 2013; Parnetti et al., 2019), while total SNCA plasma levels appear to be a less reliable diagnostic biomarker for PD, likely because of the contribution of red blood cell-derived SNCA in this matrix (Parnetti et al., 2019).