The purpose of the study is to evaluate the use of an experimental test called the Syn-One Test for identifying patients with idiopathic rapid eye movement sleep disorder (iRBD) that will progress to synucleinopathies (a type of neurogenerative disease). Individuals with iRBD have unpleasant dreams and vigorous behavior during sleep that can result in injuries. This condition often is associated with the development of synucleinopathies, such as Parkinson’s disease, dementia with Lewy bodies, pure autonomic failure and multiple system atrophy, within 10 to 15 years. Synucleinopathies share the presence of a protein called alpha-synuclein (P-SYN) deposits in the nervous system. P-SYN can be measured in the skin; the Syn-One Test measures P-SYN in skin biopsies. Results of the Syn-One Test alone will not be used to change the participant’s usual healthcare and will be compared against standard methods of testing.