TNFSF11 and neoplasm: In the osteolytic metastatic process, the interaction between the tumor cells and the BM niche leads to a “vicious cycle” (36), where bone metastatic tumor cells produce cytokines and growth factors which stimulate directly OCL maturation or indirectly induce the release of IL-6 and RANKL by OBLs or BM stromal cells (BMSCs) (16, 37).