GSTM1 and lung cancer: Significant association was observed between the combined effects of GSTM1 and GSTT1 polymorphism and lung cancer risk (− − vs. + +: OR = 1.58, 95% CI: 1.34 to 1.87, P < 0.001; − − vs. + −: OR = 1.26, 95% CI: 1.13 to 1.42, P < 0.001; − − vs.− +: OR = 1.26, 95% CI: 1.08 to 1.48, P = 0.004; recessive model: OR = 1.27, 95% CI: 1.13 to 1.42, P < 0.001; dominant model: OR = 1.33, 95% CI: 1.19 to 1.48, P < 0.001) when all the eligible studies were pooled into the meta-analysis.