Specifically, this review focused on four common clock genes found dysregulated in RA patients: circadian locomotor output cycles kaput (CLOCK), brain and muscle ARNT like-1 (BMAL1), period (PER), and cryptochrome (CRY). Of the four clock genes discussed in this review, BMAL1 and PER are the most well-studied of the affected genes. The gene discussed is PER1; the disease is rheumatoid arthritis.