We excluded 34 studies by (1) no related CYP1A2 polymorphism, (2) no cancer case–control design, (3) review articles[25,26], (4) no usable genotype data[27-30] and included 19 studies[7-11,16-18,31-41] in this meta-analysis (Figure1 and Table1). The gene discussed is CYP1A2; the disease is cancer.