SOST and plasma cell myeloma: The bidirectional effect of myeloma on bone cells is mediated by a myriad of osteoclast-activating factors, such as RANKL, macrophage inflammatory protein alpha (MIP-1 alpha), interleukin-1 (IL-1), interleukin-3 (IL-3), and tumor necrosis factor alpha (TNF-α) [89,90,91], and osteoblast-inhibiting factors like dickkopf WNT signaling pathway inhibitor 1 (DKK1), sclerostin, hepatocyte growth factor (HGF), interleukin-7 (IL-7), and TNF-α [92,93,94,95,96].