In response to DNA damage, the E2F1 transcription factor is phosphorylated at serine 31 (serine 29 in mouse) by the ATM or ATR kinases, which promotes E2F1 protein stabilization. Phosphorylation of E2F1 also leads to the recruitment of E2F1 to sites of DNA damage, where it functions to enhance DNA repair. To study the role of this E2F1 phosphorylation event in vivo, a knock-in mouse model was generated, in which serine 29 was mutated to alanine. The S29A mutation impairs E2F1 stabilization in response to ultraviolet (UV) radiation and doxorubicin treatment, but has little effect on the expression of E2F target genes. The apoptotic and proliferative responses to acute UV radiation exposure are also similar between wild-type and E2f1^S29A/S29A mice. As expected, the S29A mutation prevents E2F1 association with damaged DNA and reduces DNA repair efficiency. Moreover, E2f1^S29A/S29A mice display increased sensitivity to UV-induced skin carcinogenesis. This knock-in mouse model thus links the ability of E2F1 to directly promote DNA repair with the suppression of tumor development. Cancer Res; 74(12); 3369–77. ©2014 AACR.