GSTM1 and cancer: GSTM1 (null/present) might rise the overall cancer risks in both smokers (I2 = 68.20%, OR = 1.347, 95% CI: 1.196–1.516, P < .001) and nonsmokers (I2 = 54.70%, OR = 1.423, 95% CI: 1.270–1.594, P < .001).