Functions as a weak apurinic/apyrimidinic (AP) endodeoxyribonuclease in the DNA base excision repair (BER) pathway of DNA lesions induced by oxidative and alkylating agents. Initiates repair of AP sites in DNA by catalyzing hydrolytic incision of the phosphodiester backbone immediately adjacent to the damage, generating a single-strand break with 5'-deoxyribose phosphate and 3'-hydroxyl ends. Also displays double-stranded DNA 3'-5' exonuclease, 3'-phosphodiesterase activities. Shows robust 3'-5' exonuclease activity on 3'-recessed heteroduplex DNA and is able to remove mismatched nucleotides preferentially. Shows fairly strong 3'-phosphodiesterase activity involved in the removal of 3'-damaged termini formed in DNA by oxidative agents. In the nucleus functions in the PCNA-dependent BER pathway. Required for somatic hypermutation (SHM) and DNA cleavage step of class switch recombination (CSR) of immunoglobulin genes. Required for proper cell cycle progression during proliferation of peripheral lymphocytes. Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Probably binds two magnesium or manganese ions per subunit. 3'-5' exonuclease activity is activated by sodium and manganese. 3'-5' exonuclease and 3'-phosphodiesterase activities are stimulated in presence of PCNA. Optimum pH is 6.0-8.0. Interacts with PCNA; this interaction is triggered by reactive oxygen species and increased by misincorporation of uracil in nuclear DNA. Together with PCNA, is redistributed in discrete nuclear foci in presence of oxidative DNA damaging agents. Highly expressed in brain and kidney. Weakly expressed in the fetal brain. Belongs to the DNA repair enzymes AP/ExoA family.