DNA polymerase beta (POLB) catalyzes DNA strand displacement synthesis. In this process, 2-10 nucleotides (dNMPs) are incorporated at the 3' end of the single strand break (SSB). Simultaneously, the other broken DNA strand, which contains a 5' oxidatively-damaged AP dideoxyribose phosphate residue (5'ddRP), is displaced as a flap structure. POLB-mediated DNA strand displacement synthesis is facilitated by PARP1 and/or PARP2 dimers, which bind the single strand DNA (ssDNA) ends and FEN1 (flap endonuclease) (Klungland and Lindahl 1997, Prasad et al. 2001, Lavrik et al. 2001, Liu et al. 2005).