SLC6A3 and Parkinson disease: A recent series of clinical studies has demonstrated elevated serum bilirubin levels in Parkinson’s disease (PD) patients (Moccia et al., 2015; Lee et al., 2019; Macias-Garcia et al., 2019; Jin et al., 2020), and total serum bilirubin had a positive correlation with presynaptic dopamine transporter uptake (Lee et al., 2019).