Striatal dopamine transporter (DAT) imaging using 123I-ioflupane single photon positron emitted computed tomography (SPECT), also called DaTScan (GE Healthcare, Chicago, IL, USA), and 18F-fluorodopa positron emission tomography (PET) have found that between 5 and 20% of newly diagnosed PD subjects enrolling in clinical PD studies have scans without evidence of dopaminergic deficit (SWEDD) (Batla et al., 2014; Erro et al., 2016). This evidence concerns the gene SLC6A3 and Parkinson disease.