GSTM1 and head and neck squamous cell carcinoma: The present meta-analyses of 42 studies including 7584 cases and 8651 controls for analysis of GSTM1, 32 studies including 6255 cases and 7138 controls for analysis of GSTT1, and 15 studies including 2657 cases and 3092 controls provide more comprehensive information on the relationships between two genes and HNSCC.