APEX1 and cancer: Cdk5 takes part in DDR process mainly by phosphorylating some of the critical DDR proteins such as ataxia–telangiectasia mutated (ATM) and apurinic/apyrimidinic endonuclease 1 (Ape1), and the inhibition of this kinase activity has been proven to regulate DDR process and cancer progression [11, 12].