CTSG and rheumatoid arthritis: CatG has also been reported to activate receptors, platelets, and angiotensin I, amongst others.[1, 4, 5] Importantly, the physiological activity of CatG is regulated by α2-macroglobulin, serpin B1, α1-antichymotrypsin, α1-protienase inhibitor, proteinase inhibitor 6, and secretory leukocyte protease inhibitor.[3, 6–8] Given the wide substrate specificity of CatG, it has been reported to contribute to many diseases such as periodontitis, rheumatoid arthritis, ischaemic reperfusion injury, coronary artery disease and bone metastasis.