Single photon emission computed tomography (SPECT) and positron emission tomography (PET) studies have shown that PD patients exhibit decreased levels of dopamine transporters (DATs) and vesicular monoamine transporter type 2 (VMAT2) and reduced activity of dopa decarboxylase as measured by the conversion of l-dopa to dopamine in the striatum using [18F]-fluoro-dopa PET (Felicio et al. 2009). This evidence concerns the gene DDC and Parkinson disease.