This study aims to study the natural progression of P-SYN quantification by conducting skin biopsies in patients with Parkinson's Disease (PD) and REM sleep behavior disorder (RBD). Participation will be over an 18-month period and will include skin biopsies and clinical examinations every six months. Subjects will be male or female between the ages of 50-100 and have a diagnosis of PD or RBD.