Function of AXII/AXIIR axis in MM cell adhesion and growth in the BM. Interactions between MM-derived AXIIR and OBL/stromal cell–derived AXII MM support the adhesion of MM cells in the BM. As OCL formation increases in response to OCL stimulatory factors produced or induced by MM cells, the OCLs release soluble AXII, which binds AXIIR and induces the growth of MM cells. The AXII/AXIIR axis is a crucial component of the BM microenvironment in supporting MM cells in the BM and could potentially support the homing and lodgment of MM cells in the BM.