A 317‐amino acid peptide, member of the tumor necrosis factor (TNF) superfamily, that stimulates osteoclast differentiation and activity as well as inhibiting osteoclast apoptosis. It is expressed by osteoblast‐stromal cells, fibroblasts, and activated T cells. In the bone tissue, RANKL binds directly to its receptor (i.e., receptor activator of nuclear factor‐kappa B [RANK]) on the surface of osteoclasts or preosteoclasts, stimulating both the differentiation of osteoclast progenitor cells and the activity of mature osteoclasts. It exists as either a 40–45 kD cellular, membrane‐bound form or a 31 kD soluble form derived by cleavage of the full‐length form at position 140 or 145. It also has a number of effects on immune cells, including activation of c‐Jun N‐terminal kinase (JNK) in T cells, inhibition of apoptosis of dendritic cells, induction of cluster formation by dendritic cells, and proliferation of cytokine‐activated T cells.