RRM2 and pancreatic neoplasm: The hENT1 × dCK/RRM1 × RRM2 expression ratio significantly correlates with resistance to gemcitabine in pancreatic cancer cells, including acquired gemcitabine-resistant cells, suggesting that a decrease of this ratio reflects inherent and acquired chemoresistance of pancreatic cancer cells to gemcitabine and may be a key to understanding the variable effectiveness of gemcitabine among individual patients.