GSTM1 and lip and oral cavity carcinoma: Meta‐analysis results: The meta‐analysis of four included studies of tissue samples showed a significantly (p < .001) increased GPx activity in the oral cancer group compared to the control group with the pooled standardized mean difference value of 2.572 μmol/min/g protein at 95% CI (1.495–3.650) (Figure 2), whereas three included studies of erythrocyte samples displayed a significantly (p < .001) decreased GPx activity in the oral cancer group than the control group with the pooled standardized mean difference value of −2.766 moles/min/g Hb at 95% CI (−3.297 to −2.234) (Figure 3).