Studies demonstrating that RANKL-producing osteoblasts, osteocytes, and bone marrow stromal cells [6,30] can attract and promote the migration of RANK-expressing tumor cells, such as EO771, offer a compelling explanation for the osteotropic nature of RANKL in breast cancer metastasis [5,24,31]. The gene discussed is TNFRSF11A; the disease is neoplasm.