For people with MCI, who subsequently developed PD with half of them converting to PDD during the ~5-year follow-up period, PDD risk was associated with reduced putamen-to-caudate striatal binding ratio in the less affected hemisphere with dopamine transporter single photon emission computed tomography (SPECT), coupled with hypometabolism of temporal lobes on FDG PET124. The gene discussed is SLC6A3; the disease is Parkinson disease.