On the other hand, mechanisms that impair the circadian clock genes, such as circadian locomotor output cycles kaput (CLOCK), brain and muscle Arnt-like protein 1 (BMAL1), and period genes (PER1, PER2, and PER3), contribute to defective beta-cell function and development of type 2 diabetes [7–12]. The gene discussed is PER2; the disease is type 2 diabetes mellitus.