PARP1 and cancer: The advance of inhibitors to the DDR protein PARP1 into clinical practice and the observed resistance in many cancer types to PARP1-inhibitor treatment has promoted efforts for further understanding of PARP1-inhibitor resistance, the mechanisms of action of PARP1 and ADP-ribosylation (PARylation) as well as the role of the other 16 PARP family members in cancer onset that may lead to additional cancer treatment dependencies.