First, we confirm previous reports showcasing that cathepsin S mRNA expression was significantly increased in PBMCs of active RA patients (1.34-fold increase compared to healthy controls, P < 0.05, Fig. 4A), as well as at the synovium of RA patients (6-fold increase compared to normal synovium, P < 0.001, Fig. 4B). Here, CTSS is linked to rheumatoid arthritis.