Several classes of novel CAIX inhibitors, including ureidosulfonamides (Pacchiano et al., 2011), glycosyl coumarins (Touisni et al., 2011) and indanesulfonamides (Dubois et al., 2011) have been used successfully to inhibit tumor growth in preclinical models of hypoxic, CAIX-positive breast cancer (Lou et al., 2011; Pacchiano et al., 2011; Touisni et al., 2011) and colorectal cancer (Dubois et al., 2011), demonstrating that the selective pharmacologic inhibition of CAIX activity elicits an anti-tumor effect in vivo. Here, CA9 is linked to breast cancer.