GSTM1 and chronic obstructive pulmonary disease: Thirty case–control studies met the inclusion criteria for GSTM1 (including 4443 COPD cases and 5103 controls), 27 studies for GSTT1 (including 3376 cases and 4014 controls), and 16 studies for GSTM1-GSTT1 combined effects (including 1708 cases and 1936 controls).