Fatigue and its severity, as derived from the MDS-UPDRS fatigue item, will be evaluated for correlations with other measures including the following: serial dopamine transporter imaging, blood drawn for genotyping and assessment of plasma proteins, cerebrospinal fluid, serial magnetic resonance imaging brain imaging (structural and diffusion tensor imaging), and nonmotor phenomena, such as cognitive functioning, depression, anxiety, and daytime sleepiness. Here, SLC6A3 is linked to major depressive disorder.