The results suggested that source of controls (P = 0.027 for + − vs. + +), type of controls (P = 0.005 for − − vs. + +), and quality score of articles (− + vs. + +: P = 0.045 for − − vs. + +) were source of heterogeneity between the combined effects of GSTM1 present/null and GSTT1 present/null polymorphisms with BC risk. Here, GSTM1 is linked to breast cancer.