PER2 and major depressive disorder: Of these genes, two circadian genes, period circadian regulator 3 (PER3) and PER2 were significantly upregulated in MDD-S vs. MDD-NS (PER3: p = 0.001, Q = 0.1; FC = 2.650; PER2: (p = 0.041, q = 0.207; FC = 1.871).