Characteristics of 11 eligible studies.

HB: hospital-based; PB: population-based; PCR-RFLP: polymerase chain reaction-restricted fligment length polymorphism; HWE: Hardy-Winberg Equibrilium; NA: not available; NC: nasopharyngeal carcinoma; EC: esophageal carcinoma; LC: lung cancer; BC: breast cancer; SCCHN: squamous cell carcinoma of the head and neck; LSCC: laryngeal squamous cell carcinoma; HCC: hepatocellular carcinoma; a: mean + standard devation; † the study by Lu J [18] was treated as two studies, Lu Ja: test set, Lu Jb: validation set;‡ in the study by Naidu R [20], each population was considered as a study.