E2F2 and head and neck squamous cell carcinoma: Although the exact mechanism underlying E2F-related tumor susceptibility is not fully understood with conflicting data available [26, 28, 29], E2F1 and E2F2 SNPs have been shown to influence the risk of various cancers such as head and neck squamous cell carcinoma [19, 30, 31], breast [32], colon and colorectal [16, 33, 34], gastric [35], ovarian [36] and other types of cancers [30].