The major cause of mortality associated with breast cancer is the development of distant meatastases to sites such as lung and bone. Approximately 70% of patients who die from breast cancer have evidence of metastases in the skeleton. The consequences of bone metastasis are always devastating. The clinical outcomes of severe pain, pathologic fractures, spinal cord and nerve compression, leading to hypercalcemia and acid/base imbalance severely diminish the quality of life. Once tumour cells home to bone, curative therapy is no longer possible in most patients and only palliative therapy is available. This emphasises the importance of understanding the mechanisms of primary tumour cell invasion and spread to bone, to be able to identify molecular drivers of bone metastasis as new therapeutic targets. Proteases are known to contribute to tumour cell invasion and angiogenesis and are commonly associated with metastasis. Recent studies in our laboratory support a role for cysteine cathepsin proteases in bone metastasis. Using our unique 4T1.2 syngeneic model of spontaneous bone metastasis, we identified the endogenous cysteine cathepsin inhibitor stefin A as a metastasis suppressor. Tumour cell expression of stefin A significantly reduced bone metastasis in the murine model. The fact that stefin A is a potent metastasis suppressor indicates that its targets, the cysteine cathepsins, have essential roles in distant metastasis. Of the cysteine cathepsins, cathepsin B was co-expressed with stefin A in primary tumours and metastases, suggesting that the mechanism of metastasis suppression by stefin A was through inhibition of cathepsin B. Cathepsin B is highly upregulated in a wide variety of cancers, including breast and prostate cancer and is linked to enhanced tumourigenesis. In this study, using the 4T1.2 spontaneous bone metastasis model, we evaluated the function of cathepsin B in bone metastasis and the potential of selectively targeting this protease as a novel therapeutic. Cathepsin B was abundant in 4T1.2 mammary tumours and matched spine metastases, mimicking that of the human disease. We have demonstrated a critical function for tumour-derived cathepsin B in bone disease. Stable knockdown of cathepsin B in tumour cells significantly reduced collagen I degradation in vitro and bone metastasis in vivo. Additionally, use of a highly selective cathepsin B inhibitor CA-074 significantly reduced metastasis to lung and bone, a reduction that was not observed when using a broad spectrum cysteine cathepsin inhibitor. This study reveals the pro-metastatic role of cathepsin B in distant metastasis and the therapeutic benefit of its selective inhibition in a murine model of breast cancer.