Mice deficient in RANK or RANKL, the two major mediators of osteoclastogenesis, showed severe osteopetrosis along with reduced numbers of mature differentiated B cells secreting IgM and IgD in the lymph nodes and spleens, which could be related to reduced bone marrow cavities or altered stromal cells [50]. The gene discussed is TNFRSF11A; the disease is osteopetrosis.