To determine the contribution of host derived MMP-2 in mammary tumor growth in bone, two independent mammary tumor cell lines derived from the transgenic polyoma middle T antigen (PyMT) model of mammary tumorigenesis, denoted PyMT-Luc and 17L3C-Luc [19], [20], were injected into the tibia of six week old syngeneic immunocompetent FVB wild type (n = 10) and MMP-2 null (n = 10) animals. This evidence concerns the gene MMP2 and breast cancer.