Osteoblasts produce RANKL, a ligand for the receptor activator of NF-κB (RANK) on hematopoietic cells, encourages the stimulation osteoclast activity. Osteoblasts also produce osteoprotegerin which blocks RANKL/RANK cycle, in charge of osteoclast activity and stimulation. Osteoblasts interact with osteoclasts in order to activate RANK. So, it has been determined that excessive expression of osteoprotegerin, which increase with age, are correlated with an increased risk of fractures and a lowering in bone mass density.