The Parkinson's Progression Markers Initiative (PPMI) is a landmark observational study of 400+ newly diagnosed PD patients followed longitudinally with clinical, imaging, and biomarker data. CSF alpha-synuclein, DJ-1, phospho-tau, and neuroimaging endpoints. Essential reference for PD biomarker studies and gut-brain axis research. Accessed via the PPMI data portal (www.ppmi-info.org).