Carbonic anhydrases (CAs) are a large family of zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide. Carbonic Anhydrase 9 (CA9) has a distinctive expression pattern in normal and cancer tissues. The most abundant expression of CA9 was found in normal mucosa of the stomach and gallbladder. Other normal tissues have lower or no expression. Relatively high levels of CA9 are expressed in carcinomas of the cervix, kidney, lung, breast and many other tumors. Most studies have shown that decreased CA9 levels are independently associated with poor survival. Low levels of CA9 may benefit more from adjuvant treatment than patients with high levels.