Osteoprotegerin is a protein that plays a central role in regulating bone mass. Throughout life, bone is constantly being broken down and rebuilt in a process that replaces old bone with new bone. These events are mediated by osteoclasts, the cells that resorb bone, and osteoblasts, the cells that form bone. Maintaining bone mass is critical for the integrity of the skeleton, which provides protection for soft tissues, support for locomotion, a scaffold for muscle attachment, and a reservoir for mineral ions (calcium, phosphorus, magnesium).