PER2 and rheumatoid arthritis: In addition, the DAS28 erythrocyte sedimentation rate (ESR) was negatively correlated with the expressions of PER2. Researchers concluded that the expression of the clock genes, specifically PER2, E-4 binding protein 4 (E4BP4), and RORA, serves as useful biomarkers in predicting RA disease activity and the efficacy of DMARDs in RA patients.