We utilized the 4T1 murine breast cancer model in which we have demonstrated a critical role of CAIX in metastasis formation.9 Importantly, the 4T1 cell line is a well-established, tractable syngeneic model of CAIX-positive metastatic breast cancer and offers the distinct advantage of allowing for shRNA-mediated depletion of endogenous murine CAIX in combination with constitutive expression of human CAIX, which is resistant to shRNA directed toward the murine isoform. Here, CA9 is linked to breast carcinoma.