Auto-PARylated (poly(ADP-ribosylated)) PARP1 or PARP2 homodimers and heterodimers dissociate from DNA at single strand breaks (SSBs). The dissociation of PARP1 and/or PARP2 is necessary for completion of the long patch base excision repair (BER). In the presence of PARP inhibitors, PARP1 and/or PARP2 cannot auto-PARylate and remain bound to SSBs. This leads to persistence of SSBs, and generation of double strand breaks (DSBs) during DNA replication (Creissen and Shall 1982, Sukhanova et al. 2004, Sukhanova et al. 2005, Pachkowski et al. 2009, Heacock et al. 2010, Strom et al. 2011).