Exclusion criteria were any current or past clinically significant medical or neurological illness (other than PD) that could affect study outcome, cognitive impairment (as determined by a cut-off of <21 on the Montreal Cognitive Assessment (MoCA) test19), a history of alcohol or substance abuse, medication affecting SV2A binding (e.g., levetiracetam), current pregnancy (as documented by pregnancy testing at screening and on the day of PET imaging), breast feeding, and contraindications to magnetic resonance imaging (MRI). The gene discussed is SV2A; the disease is substance abuse.