Migrating tumor cells produce a protein that aids them to set up home in bones, researchers show. If a tumor develops metastases, the chances of the patient’s survival will be severely diminished. Cancer cells that leave the primary tumor, travel through the body, and set up home in distal organs such as lungs and bones start to express cathepsin K. Cathepsin K is primarily found only in the bone and is secreted by osteoclasts.