Parkinson's disease (PD) is the second most common age-onset neurodegenerative brain disorder. The disease manifests itself as complications with movement, and in about one third of the cases, is followed by cognitive impairment and dementia. The onset and severity of Parkinson's disease is associated with the presence and distribution of proteinaceous aggregates called Lewy bodies (LBs). Sadly, the presence of LBs can be determined only post-mortem by a pathologist. Therefore, we are proposing to develop an LB assay based on biofluid protein measurements. Our approach is based on two linked hypotheses. The first is that there are proteins and/or their post-translational modifications that increase in abundance in the presence of LBs. The second hypothesis is that proteins that infiltrate the cerebrospinal fluid and blood from the brain will b present in low abundances. Regardless the specific mechanism of brain to biofluid infiltration, we anticipate that it is a low efficiency process and requires highly sensitive proteomics approaches to detect specific neurons in the cerebrospinal fluid and especially in the blood. To test these hypotheses we have designed a two-stage study comprised of three objectives: 1) Discover brain tissue specific proteins and their modifications that correlate with the presence of LBs, using LC-MS proteomics data obtained for a group of 500 individuals from Religious Orders Study and Memory and Aging Project studies conducted by Rush University. Differentially abundant proteins exclusively expressed in the brain tissue will used for follow-up testing in biofluids. 2) Using ultra-sensitive selected reaction monitoring approach we will verify the correlation of abundances for these selected markers in the brain tissue and biofluids, including both cerebrospinal fluid and blood serum. We will then apply this information to develop a biofluid-based statistical model that predicts the LB load. 3) Validate the model by acquiring blood serum measurements for an independent PD case/control cohort based on the subjects enrolled into the Osteoporotic Fractures in Men study.