In osteolytic metastasis produced by a large majority of breast cancer cells, the osteoclast activity is stimulated by cancer cells that release interleukin-1 (IL-1), IL-6, PTHrP, prostaglandin E2 (PEG2), TNFalpha (Tumor Necrosis Factor), VEGF (Vascular Endothelial Growth Factor), and M-CSF (Figure 1). The gene discussed is PTHLH; the disease is breast cancer.