CA9 and cancer: Prior to the identification of CA isoform(s) mediating the cytotoxic effects of A6 and A15, we first examined the protein levels of several CA isoforms, including ubiquitously expressed cytosolic CA I and II and cancer-associated transmembrane CA IX and XII, in the presence or absence of CoCl2 in MCF-7 and SK-BR-3 cells using western blotting.