The purpose of this study is to assess the ability of the Syn-One Test to measure the amount of phosphorylated alpha-synuclein (P-SYN) in skin biopsies from patients with Parkinson’s disease (PD) or rapid eye movement sleep behavior disorder (RBD). Researchers will also evaluate the ability of skin biopsies to measure changes in P-SYN deposits over time. The study will include both people who have been diagnosed with PD and those who have been diagnosed with RBD without evidence of PD. The Syn-One Test measures the amount P-SYN in a skin punch biopsy. By analyzing skin punch biopsies, P-SYN can be identified within nerve fibers of the skin which can help with the diagnosis of neurological disorders. Accurate and early detection of may improve the ability to start treatment earlier.