In the bone microenvironment, breast cancer cells produce osteoclast (OC)-activating cytokines, including parathyroid hormone-related protein (PTH-rP), prostaglandin E2 (PEG2), and interleukin-11 (IL–11) [5], which can increase expression of receptor activator of nuclear factor-κB ligand (RANKL) by osteoblasts (OBs). The gene discussed is TNFSF11; the disease is breast carcinoma.