GSTM1 and actinic keratosis: Catalase activity levels also showed a significant association with cancer types (p < 0.001); the mean catalase activity value of the SCC group (179.32 U/mg Hb) was significantly lower than that of the actinic keratosis group (206.87 U/mg Hb, p = 0.007) and the controls (216.54 U/mg Hb, p < 0.001) (Figure 2).