Base excision repair (BER) is the predominant system correcting simple DNA base lesions formed by oxidation or other DNA damaging agents. Repair of apurinic/apyrimidinic (AP) sites arising in the genome spontaneously or as intermediates of BER is critical owing to their toxic and mutagenic effects. The mammalian apurinic/apyrimidinic endonuclease APE1 (EC 4.2.99.18) is a Mg2+-dependent multifunctional protein operating in protection of cells from oxidative stress via its DNA repair, redox, and transcription regulatory activities. The human AP endonuclease APE1, also called Ref-1 is the major and crucial enzyme for the recognition and processing of AP sites in the base excision repair (BER) of DNA. APE1 operates by incising the DNA phosphodiester backbone 5′ to AP sites, generating a nick with 3′-hydroxyl and 5′-deoxyribose phosphate (dRP) termini. Repair of the resulting nick is completed by DNA polymerase and DNA ligase. In addition to its endonuclease activity, APE1 is known to have 3′-phosphodiesterase and 3′-phosphatase activity and 3′ to 5′ exonuclease activity as well as a role in regulating the redox state of several transcription factors.