Clinical data have demonstrated that dopamine and dopamine transporter (DAT) detected by single photon emission computerized tomography (SPECT) or positron emission tomography (PET) promise to be objective and non-invasive markers to identify and determine retinopathy in PD patients (Nguyen-Legros, 1988; Wojtkowski et al., 2004; Biehlmaier et al., 2007). Here, SLC6A3 is linked to Parkinson disease.