Receptor activator of NF-κB (RANK) ligand (RANKL) is type II transmembrane protein with an extracellular domain at the carboxy terminus of TNF cytokine superfamily. RANKL is a 19.8 kDa protein containing 317 residues and high expressed in T cells and T cell rich organs, such as thymus and lymph nodes. RANKL-RANK (RANKL receptor) plays an important role in bone metabolism, dysregulation, and immune system. RANKL deficiencies in mice or humans are associated with abnormally increase bone density and blemish in lymphoid organogenesis.