Background: Carbonic anhydrase IX (CA9/CA IX) is a zinc-containing metalloproteinase mainly distributed in the gastrointestinal tract and gallbladder. CA IX is overexpressed in many types of cancer, including renal clear cell carcinoma (RCC) as well as cervical, breast, and lung cancers, but is lowexpressed in normal kidney and most other normal tissues. It is mainly used in the diagnosis of clear cell renal cell carcinoma. 75-100% of clear cell renal cell carcinoma is diffusely and strongly positive, and other types of renal cell carcinoma are negative or only focally positive.