CLOCK and major depressive disorder: Certain genetic variants of clock genes, including BMAL1 rs900147, PER1 rs2253820 (Gu et al., 2015), CLOCK 3111T/C (Lou et al., 2018), and Tef rs738499 (Hua et al., 2012), have been linked to specific PD symptoms such as tremors, motor fluctuations, sleep disorders, and depression, respectively.