CSF1 and neoplasm: As for osteoclasts, their interaction with tumor cells is reciprocal: tumor cells produce factors (e.g., parathyroid hormone-related peptide; interleukin-6; tumor necrosis factor; and macrophage colony stimulating factor, M-CSF) that directly or indirectly induce the formation of osteoclasts, and activated osteoclasts produce factors (e.g., transforming growth factor, TGF-β; insulin growth factor, IGF; and bone morphogenetic proteins, BMPs) that stimulate tumor growth and bone destruction [1].