TNFSF11 and neoplasm: In bone metastasis, tumor cell-secreted CST6 enters osteoclasts and inhibits the activity of the cysteine protease cathepsin B (CTSB), leading to the up-regulation of sphingosinekinase1 (SPHK1), which inhibits RANKL-induced activation of p38 and suppresses osteoclast maturation (174) (Figure 2).