Polymerase Chain Reaction (PCR) and Restriction Fragment Length Polymorphism (RFLP) methods were used to measure DNA damage repair gene XPC and folate metabolism enzyme gene MTHFR polymorphisms.

• 对照组选自同期居住在扬中境内的居民，与病例按性别和年龄组进行成组频数匹配，要求身体健康，无恶性肿瘤及胃和十二指肠溃疡病史，征得同意后采集静脉血并进行流行病学问卷调查。