RANKL and RANK are members of the TNF superfamily of ligands and receptors that play an important role in the regulation of specific immunity and bone turnover. RANK (receptor) was originally identified as a dendritic-cell-membrane protein, which by interacting with RANKL augments the ability of dendritic cells to stimulate naïve T cell proliferation and to promote the survival of RANK + T cells. RANK is also expressed in a variety of tissues including skeletal muscle, thymus, liver, colon, small intestine and adrenal gland. The RANK/RANKL interaction is important in the regulation of osteoclastogenesis and in dendritic-cell-mediated T cell immune responses. Impairments in RANK signaling have been implicated in the induction of expansile osteolysis and Paget disease of bone (PDB2). Recombinant human sRANK receptor is a 19.3 kDa polypeptide containing the TNFR homologous cysteine rich portion of the extracellular domain of RANK receptor (175 amino acid residues).