PARP1 and breast cancer: In addition, absence of CDK12 protein is often accompanied by downregulation of DDR proteins (ATR (ataxia-telangiectasia and Rad3-related), Ku70/Ku80 (the classical non-homologous end joining (cNHEJ) factors), PARP1 (poly ADP-ribose polymerase 1), DNA-PK (DNA-dependent protein kinase) and γH2AX (phosphorylated histone H2AX)), suggesting a novel mechanism of CDK12-associated DDR dysregulation in breast cancer [61].