SNCA and Parkinson disease: The objective of the Synuclein-Quantification (Syn-Q) study is to quantify cutaneous P-SYN across a range of PD severity and in iRBD and measure changes in P-SYN over time.<h4>Methods</h4>After consent, participants with iRBD and PD across Hoehn and Yahr stages 1, 2, 3 will complete neurological and cognitive evaluation, motor and olfactory assessments, orthostatic vitals, and questionnaires.