Data extraction
Two authors (AP and NN) independently extracted the relevant data. The extracted results were checked by a third author (ARP). The following items were explored in eligible articles: author’s first name, publication’s year, country, ethnicity, source of group, total number of cases and controls, genotyping methods, cancer types, genotype distribution frequencies, minor allele frequency (MAF) and P value for Hardy-Weinberg equilibrium (HWE) in the control group. The quality of the included studies was assessed by two authors independently according to the modified Newcastle-Ottawa scale (NOS) (supplementary table S1). The scores ranged from 0 (the lowest quality) to 10 points (the highest quality) (Goto et al.). Each type of cancer, source of control, and genotyping method with two or more related reports were included in the subgroup analysis.