CA9 and tongue squamous cell carcinoma: We have previously shown that carbonic anhydrase IX (CA9) expression is increased in the pingyangmycin (PYM)-induced multidrug-resistant human tongue squamous cell carcinoma cell line Tca8113/PYM when compared to the parental cell line Tca8113, without significant expression difference in multidrug resistance genes such as MDR, BCRP and MRP.