 Thus, the RANKRANKL system is vital for activation of the bone resorping cells osteoclasts. In the skeleton the bone synthesizing cells osteoblasts express RANKL that signals to RANK on the immature osteoclasts. This induces proliferation and activation of the cells they start to proliferate and resorp bone. OPG is produced by somatic cells in the bone and this production is regulated by sex hormones, TGF-B and various other substances. Today a human made recombinant antibody against RANKL, Denosumab is used to treat osteoporosis as it inhibits RANKL signalling and causes less bone resorption in humans. RANKL signalling has only two other known additional functions in healthy humans where it is involved in lactation and the immune response. 