OPG knockout mice exhibit osteoporosis due to excessive bone reabsorption (Bucay et al., 1998), and conversely, elevated OPG levels or inactive RANKL result in osteopetrosis due to reduced bone reabsorption (Simonet et al., 1997, Vitovski et al., 2007). This evidence concerns the gene TNFRSF11B and osteoporosis.