In summary, our study shows a unique UGT1A genotype pattern in Korean gastric cancer patients and suggests that the presence of the UGT1A1*6, UGT1A7*3 and UGT1A9*22 polymorphisms in gastric cancer patients receiving irinotecan-containing treatment was significantly associated with severe toxicity. The gene discussed is UGT1A9; the disease is gastric cancer.