Parathyroid hormone–related protein (PTHrP; encoded by Pthlh) is produced by breast cancer cells and increases RANKL (encoded by Tnfsf11) in the bone while reducing osteoprotegrin (OPG; encoded by Tnfrsf11b), thereby shifting the RANKL to OPG ratio to establish a vicious cycle in favor of increased bone resorption (5). The gene discussed is TNFSF11; the disease is breast carcinoma.