Importantly, both Cathepsin K, a potent collagenase typically expressed by osteoclasts during osteolysis, and MMP-9 were reported to be induced upon HA-mediated CD44 activation in prostate and breast cancer cells, suggesting their role in the colonization of metastatic osteolytic prostate and/or breast cancer cells (58–60). Here, MMP9 is linked to breast cancer.