In vivo, mouse studies have shown that MMP-13, a “classic collagenase” almost universally upregulated across malignancies (Gobin et al., 2019), promotes mammary tumor-induced osteolysis by activating MMP-9 and increasing TGF-β signaling at the tumor-bone interface (Nannuru et al., 2010). This evidence concerns the gene MMP9 and breast cancer.