Carbonic Anhydrase (CA IX) is part of a family of zinc containing metalloproteins that catalyze the reversible hydration of CO₂. Among these, CA IX is anchored to the cell membrane and is expressed in the human gastrointestinal tract, chiefly in the stomach.¹ Data suggest consistent immunoreactivity for anti-CA IX in clear cell renal cell carcinoma (RCC) making it a useful marker for this type of tumor.