Ape1 initiates the repair of abasic sites and is essential for the base excision repair (BER) pathway. Repair activities of Ape1 are stimulated by interaction with XRCC1, another essential protein in BER. Ape1 functions as a redox factor that maintains transcription factors in an active, reduced state but can also function in a redox-independent manner as a transcriptional cofactor to control different cellular fates such as apoptosis, proliferation and differentiation.