Parkinson’s disease (PD) remains a diagnosis that is made primarily upon clinical grounds, and there is, as yet, no definitive diagnostic laboratory test. Unfortunately, clinical diagnosis can be challenging, since signs and symptoms of PD overlap with other distinct neurological disorders and may be masked by symptomatic treatment. Moreover, PD has a heterogeneous presentation and insidious onset, which makes early diagnosis particularly susceptible to error. A valid diagnostic biomarker for PD would remove several roadblocks to improving patient outcomes. First, it would improve diagnostic accuracy. Second, it would allow earlier diagnosis, which is critically important if disease-modifying and/or neuroprotective approaches are available. Third, it would better define diagnostic subtypes, which would reduce heterogeneity of subjects for enrollment into PD research studies and might provide appropriate individualized therapeutic approaches (Kang 2009, Michell et al. 2004).