SOD2 and esophageal cancer: Several studies have found the associations between the Val form of the MnSOD gene and increased cancer risk [7–9], but a majority of studies showed the Ala form to be associated with higher cancer risk, such as breast cancer [10, 11], esophageal cancer [12], colorectal cancer [13], and cervical cancer [14], and some other studies find no significant association between this polymorphism and cancer risk [15–18].