CTSK and rheumatoid arthritis: The most important proteinases involved in bone destruction are cathepsins B, L, and K, which are upregulated in synovial fibroblasts in RA patients.73 Cathepsin K, a cysteine proteinase, is crucial in bone remodelling and is predominantly expressed in osteoclasts as well as in fibroblasts and macrophages in RA joints.74 Recent animal studies have shown that genetic deletion of cathepsin K causes significant reduction in inflammation and bone erosion within RA joints.75