CA9 and cancer: Thus, CAIX is an attractive target for cancer diagnosis and therapy for the following reasons: (1) CAIX expression in tumors is upregulated and associated with poor prognosis; (2) CAIX is overexpressed in a wide variety of solid tumors, such as renal cell carcinoma, brain, bladder, cervical, head and neck, breast, lung, and kidney cancers; (3) CAIX expression in normal tissues is highly restricted; (4) CAIX is easily accessible for drug targeting due to its location on the cell membrane.