Homodimers and/or heterodimers of PARP1 and PARP2 bind single strand DNA (ssDNA) ends along with FEN1 (flap endonuclease), forming a ternary complex with POLB (DNA polymerase beta) and simultaneously displacing APEX1 (Prasad et al. 2001, Lavrik et al. 2001, Cistulli et al. 2004). While PARP2 is much less catalytically active than PARP1 in DNA damage-induced poly(ADP-ribosyl) (PAR) synthesis (Shieh et al. 1998, Ame et al. 1999, Fisher et al. 2007), the functional redundancy between PARP1 and PARP2 is probably important. Knockout of both PARP1 and PARP2 homologs is embryonic lethal in mice, while knockout of individual PARPs is not (Menissier de Murcia et al. 2003).