Clinical studies have shown that striatal dopamine is relatively spared in patients with the tremor-dominant PD, that rest tremor is related more to serotonergic raphe nuclei deficiency than striatal dopamine (35), and that the severity of resting tremor is also related more to reduced raphe serotonin transporter than dopamine transporter (6, 36). This evidence concerns the gene SLC6A3 and Tremor.