MMP2 and breast cancer: Although mature osteoclasts were largely negative for MMP-2 expression by immunohistochemistry (Fig. 1), it is possible that MMP-2 may be expressed in early osteoclast precursors and therefore, MMP-2 could impact mammary tumor growth-induced osteolysis by affecting a) migration/recruitment of osteoclast precursors and/or b) osteoclastogenesis.