There are numerous mechanisms by which cells defend themselves against oxidants, forming a defense system mainly comprising antioxidant enzymes. SODs are generally regarded as the first line of antioxidant enzymes that protect cells against ROS, and may contribute to protection against carcinogenesis or tumor progression (28). Another antioxidant enzyme is GSH-Px, which is also a significant H2O2-scavenging enzyme (29). Although a number of reports have studied the level of SOD and GSH-Px in humans and animal cancer types, the data are conflicting and controversial (30,31). The plausible explanations for this include: i) the fact that treatment strategies that are effective in one type of cancer may be ineffective in another type, and ii) the differences between the cancerous tissues studied.