The importance of these molecules in osteoclasts was demonstrated by the findings that mice deficient for RANKL or RANK lack osteoclasts and show severe osteopetrosis14, and mice deficient for c-Fos and naturally occurring Mitf mutant mice (mi/mi mouse) also show osteopetrosis by halting osteoclast differentiation and/or fusion17, 18. The gene discussed is TNFSF11; the disease is osteopetrosis.