One of the DNA repair machineries is activated by Poly (ADP-ribose) Polymerase 
(PARP) enzyme. Particularly, this enzyme is involved in repair of damages to 
single-strand DNA, thus decreasing the chances of generating double-strand 
breaks in the genome. Therefore, the concept to block PARP enzymes by PARP 
inhibitor (PARPi) was appreciated in cancer treatment. PARPi has been designed 
and tested for many years and became a potential supplement for the conventional 
chemotherapy. However, increasing evidence indicates the appearance of the 
resistance to this treatment. Specifically, cancer cells may acquire new 
mutations or events that overcome the positive effect of these drugs. This paper 
describes several molecular mechanisms of PARPi resistance which were reported 
most recently, and summarizes some strategies to reverse this type of drug 
resistance.