The exofacial CA activity in CA9 cells was comparable to endogenous levels of membrane CA activity in wild-type MDA-MB-468 breast cancer cells, a cancer cell line known to constitutively express CAIX (Fig. 2b), which confirms that CAIX-transfected HCT116 cells have exofacial CA activity in the physiological range. The gene discussed is CA9; the disease is breast carcinoma.