Cathepsin B (CSTB), part of the papain family of proteases, is a widely expressed lysosomal cysteine endopeptidase . Cathepsin B is produced from a larger precursor form, pro-cathepsin B, which runs at approximately 44 kDa on SDS-PAGE, and is proteolytically processed and glycosylated to form a mature two-chain protein containing a heavy chain (running at 27 and 24 kDa) and a light chain (5 kDa). High levels of cathepsin B are found in macrophages and osteoclasts, as well as various types of cancer cells, including lung, colon, prostate, breast, and stomach. In addition, expression of cathepsin B has been associated with multiple sclerosis , rheumatoid arthritis , and pancreatitis . While generally localized to lysosomes, in cancer alterations can lead to its secretion . Its role in tumor progression is thought to involve promotion of basement membrane degradation, invasion and metastasis. Expression can correlate with poor prognosis for a variety of forms of cancer .