This assertion is supported by the fact that rare metastases are observed in RANKL null mutant mice injected with MOS-J PG1 cells (Figure S5), and by the observation that RANKL inhibition through a blocking antibody is capable of reversing the metastatic effect favored by RANK expression in osteosarcoma cells in an immune-compromised context. The gene discussed is TNFRSF11A; the disease is osteosarcoma.