Poly(ADP-ribose) polymerases(PARP) synthesize the ADP-ribose polymers onto 
proteins and play a role in DNA repair. PARP inhibitors block the repair of 
single-strand breaks, which in turn gives rise to double-strand breaks during 
DNA replication. Thus, PARP inhibitors elicit synthetic lethality in cancer with 
BRCA1/2 loss-of-function mutations that hamper homologous recombination repair 
of double-strand breaks. Olaparib, the first-in-class PARP inhibitor, was 
approved for treatment of BRCA-mutated ovarian cancer in Europe and the United 
States in 2014. Other PARP inhibitors under clinical trials include rucaparib, 
niraparib, veliparib, and the "PARP-trapping" BMN-673. BRCA1/2 sequencing is an 
FDA-approved companion diagnostics, which predicts the cancer vulnerability to 
PARP inhibition. Together, synthetic lethal PARP inhibition is a novel promising 
strategy for cancer intervention even in cases without prominent driver 
oncogenes.