Several studies have shown association of clock gene polymorphisms with mood disorders, including associations of: MDD with Per2, Cry1, and Rora; seasonal affective disorder with Bmal1, Per2, Cry2, and Npas2; and bipolar disorder with Clock, Bmal1, Per3, Rev-erbα, and Rorb (McCarthy and Welsh, 2012). Here, PER2 is linked to major depressive disorder.