RANKL, the receptor activator of nuclear factor kappa B ligand, a member of the tumor necrosis factor (TNF) family (http://www.uniprot.org/uniprot/O14788), is the main stimulatory factor for the formation of mature osteoclasts and is essential for their survival. RANKL activates its specific receptor RANK, located on osteoclasts and dendritic cells. The effects are counteracted by Osteoprotegerin (OPG) which acts as an endogenous soluble receptor antagonist (see: OPG ELISA, Cat.No. BI-20403). The major source of RANKL are osteocytes, former osteoblasts that become embedded within the mineralized bone matrix. RANKL is a ~35 kD type II transmembrane-type protein and is cleaved to release a soluble biologically active product that forms a homotrimer.