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. The gene discussed is CTSG; the disease is metastasis.