TNFSF11 and osteopetrosis: It was, therefore, unsurprising that loss of RANKL function was associated with osteoclast-poor osteopetrosis in young patients with RANKL mutations (autosomal recessive form, OPTB2; OMIM #259710; [24]) and in Rankl null mutant mice [1,5], as well as in monkeys or mice injected with a powerful RANKL-blocking antibody [25,26,27].