Nuclease involved in single-strand and double-strand DNA break repair (PubMed:17353262, PubMed:17396150). Recruited to sites of DNA damage through interaction with poly(ADP-ribose), a polymeric post-translational modification synthesized transiently at sites of chromosomal damage to accelerate DNA strand break repair reactions (PubMed:17353262, PubMed:17396150, PubMed:21211721). Displays apurinic-apyrimidinic (AP) endonuclease and 3'-5' exonuclease activities in vitro. Also able to introduce nicks at hydroxyuracil and other types of pyrimidine base damage (PubMed:17353262, PubMed:17396150). Together with PARP3, promotes the retention of the LIG4-XRCC4 complex on chromatin and accelerate DNA ligation during non-homologous end-joining (NHEJ) (PubMed:21211721, PubMed:23689425). Interacts with LIG4 (PubMed:17396150). Interacts with PARP1 (PubMed:17396150, PubMed:17353262). Interacts with XRCC4 (PubMed:17396150, PubMed:17353262, PubMed:18077224). Interacts (via KBM motif) with XRCC5 and XRCC6; promoting recruitment to DNA damage sites (PubMed:17396150, PubMed:17353262, PubMed:23689425). Interacts with XRCC1 (PubMed:17353262, PubMed:17507382). Localizes to DNA damage sites (PubMed:18474613, PubMed:18172500, PubMed:21211721, PubMed:23689425). Accumulates at single-strand breaks and double-strand breaks via the PBZ-type zinc fingers (PubMed:18172500). The PBZ-type zinc fingers (also named CYR) mediate non-covalent poly(ADP-ribose)-binding. Poly(ADP-ribose)-binding is dependent on the presence of zinc and promotes its recruitment to DNA damage sites. The KBM (Ku-binding motif) mediates interaction with XRCC5/Ku80 and XRCC6/Ku70 and recruitment to DNA damage sites. The FHA-like domain mediates interaction with XRCC1 and XRCC4. Poly-ADP-ribosylated. In addition to binding non covalently poly(ADP-ribose) via its PBZ-type zinc fingers, the protein is also covalently poly-ADP-ribosylated by PARP1. Phosphorylated in an ATM-dependent manner upon double-strand DNA break. Belongs to the APLF family.