Osteolysis is based on a self-perpetuating signaling system (vicious cycle) that is maintained by mitogenic factors for tumor cells such as transforming growth factor-β, insulin-like growth factor-1, fibroblast growth factors, platelet derived growth factors, and Ca-ions released from demineralized bone as well as parathyroid hormone-related peptide (PTHrP) derived from tumor cells. The gene discussed is PTHLH; the disease is neoplasm.