Colonic Tissue Biopsy Detection of Phosphorylated Alpha-synuclein for Parkinson's Diagnosis or REM Sleep Behavior Disorder

This study is looking at whether a colon tissue biopsy taken during a routine colonoscopy can detect an early marker of Parkinson's disease called phosphorylated alpha-synuclein. The goal is to find a better way to diagnose Parkinson's disease and a related sleep disorder called REM Sleep Behavior Disorder (RBD). **You may be eligible if...** - You are 40 to 99 years old and in relatively good health - You have been diagnosed with Parkinson's disease OR have been diagnosed with REM Sleep Behavior Disorder (RBD) without Parkinson's - You are already scheduled for a routine colonoscopy (for cancer screening or other digestive health checks) **You may NOT be eligible if...** - You are taking blood thinners (though aspirin alone is typically fine) - You are currently being treated for colon cancer - You have an active gastrointestinal illness - You have had recent abdominal surgery - You are pregnant or planning to become pregnant before your colonoscopy - You have significant memory or cognitive problems Talk to your doctor to see if this trial is right for you.