CA9 and breast cancer: 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.