PER3 and rheumatoid arthritis: In a 2015 published study, five blood samples from five RA (DAS28 ≥ 4.2) and five blood samples from non-RA healthy controls were evaluated [23]. Samples were collected every two hours for 24 hours and analyzed for clock gene expression in isolated CD14+ monocytes through quantitative RT-PCR. The results showed that clock genes, PER2 and PER3, in CD14+ monocytes did not exhibit circadian expression amplitude in patients with RA when compared to controls.