GSTM1 and breast cancer: The pooled estimates showed an significant association between the combined effects of both GSTM1 and GSTT1 null genotypes on BC risk (− + vs. + +: OR = 1.18, 95% CI = 1.03–1.35, − − vs. + +: OR = 1.65, 95% CI = 1.31–2.07, (− +) + (+ −) vs. + +: OR = 1.17, 95% CI = 1.05–1.30, (− +) + (+ −) + (− −) vs. + +: OR = 1.27, 95% CI = 1.12–1.43, − − vs. (− +) + (+ −) + (+ +): OR = 1.41, 95% CI = 1.19–1.68) across overall analysis.