NQO1 and lung cancer: After combining all qualified studies, we found null association of NQO1 gene C609T polymorphism with lung cancer under both allelic (OR = 0.99; 95% CI: 0.92–1.06; P = 0.692) and dominant (OR = 0.98; 95% CI: 0.89–1.08; P = 0.637) models, and this association suffered from significant evidence of heterogeneity between studies (allelic and dominant models: I2 = 32.1% and 43.7%) (Figure 2).