An association between Gaucher disease and parkinsonism has been demonstrated by the concurrence of parkinsonian manifestations in patients with Gaucher disease and an increased incidence of glucocerebrosidase (GBA) mutations in subjects with parkinsonism of various ethnicities. Furthermore, there is a significant number of obligate and confirmed Gaucher carriers with parkinsonian manifestations. Thus, alterations in GBA appear to be a more common risk factor than previously thought for the development of sporadic Parkinson disease and related disorders. However, in affected and at-risk individuals, the identification and characterization of early parkinsonian manifestations, and the rate of progression of symptoms have not been studied objectively. We propose an in-vivo study that will employ multiple imaging modalities to better define the phenotype in GBA-associated parkinsonism, follow disease progression, and identify at-risk individuals. The subjects include patients with Gaucher disease and Gaucher carriers with parkinsonian manifestations, and clinically unaffected but at-risk individuals carrying GBA mutations, and have/ a first degree relative with parkinsonism. The control groups consist of individuals with Gaucher disease but no parkinsonian symptoms, relatives of probands without GBA mutations, PD patients without GBA mutations, and healthy volunteers. Positron emission tomography (PET) with 6-[F-18] Fluoro-L-Dopa (6FD) will be used to evaluate presynaptic dopaminergic function, where 6FD uptake in putamen and striatum will be employed as a measure of neuronal structural integrity in substantia nigra. Each subject will be screened with an MRI to rule-out anatomic brain abnormalities, and to further delineate areas of interest in the PET scans. Subjects will also undergo transcranial ultrasonography (TCS) to assess the substantia nigra non-invasively. The results of both the PET scans and TCS will be kept confidential, and will not be communicated to the individuals or families involved in the study. In addition to the imaging studies, all patients will undergo a physical and neurological examination, neurocognitive evaluation, and olfactory testing.