CTSK and neoplasm: In addition, other factors that play a role in osteolytic bone metastases are secreted or induced by metastatic tumour cells (IL-1, prostaglandin E2, granulocyte macrophage colony stimulating factor GM-CSF, tumour necrosis factor-alpha TNFα, matrix metallo protein MMPs, cathepsin K CTSK and osteopontin OPN) [54,58,59,60,61,62,63].