The receptor activator of nuclear factor-kB ligand (RANKL), member of the tumor necrosis factor family, was found to be highly expressed on Treg isolated from tumor-bearing hosts, and substantial evidence indicates that RANKL expressed by Treg is involved in the onset of metastasis in both mammary (24) and prostate tumors (25). The gene discussed is TNFSF11; the disease is neoplasm.