PER2 and rheumatoid arthritis: Researchers found that RA patients’ synovial cells, when compared to controls, did not have a significant difference in PER2 protein expression, but, when exposed to LPS to induce inflammation, PER2 protein expression declined in a time-dependent manner. Their results suggested that the PER2 gene may be a risk factor for RA, and expression of the PER2 protein may be affected by inflammation [22].