Long-patch base excision repair (BER) can proceed through PCNA-dependent DNA strand displacement synthesis by replicative DNA polymerases - DNA polymerase delta complex (POLD) or DNA polymerase epsilon (POLE) complex. The PCNA-dependent branch of long-patch BER may occur in cells in the S phase of the cell cycle, when the replication complexes that contain PCNA, POLD or POLE, RPA and RFC are available. POLB incorporates the first nucleotide at the 3'-end of APEX1-generated single strand break (SSB), thus displacing the damaged AP (abasic) dideoxyribose phosphate residue at the 5'-end of SSB (5'ddRP). PCNA is recruited to BER sites by APEX1 and flap endonuclease FEN1, and loaded onto damaged DNA by RFC. POLD and POLE in complex with PCNA continue the displacement DNA strand synthesis. FEN1 cleaves the displaced DNA strand with the AP residue (5'ddRP), and DNA ligase I (LIG1) ligates the multiple nucleotide patch at the 3' end of the SSB with the FEN1-processed 5'-end of the SSB (Klungland and Lindahl 1997, Stucki et al. 1998, Dianov et al. 1999, Matsumoto et al. 1999, Podlutsky et al. 2001, Dianova et al. 2001, Ranalli et al. 2002).