Tumor necrosis factor ligand superfamily member 11 (CD254) is a cytokine that binds to TNFRSF11B/OPG and to TNFRSF11A/RANK. CD254 augments the ability of dendritic cells to stimulate naive T-cell proliferation. CD254 is an important regulator of interactions between T-cells and dendritic cells and plays a role in the regulation of the T-cell-dependent immune response. CD254 also plays a role in enhanced bone-resorption in humoral hypercalcemia of malignancy. Defects in CD254 are the cause of osteopetrosis autosomal recessive type 2 (OPTB2), also known as osteoclast-poor osteopetrosis. Osteopetrosis is a rare genetic disease characterized by abnormally dense bone, due to defective resorption of immature bone.