CDK5 participates in the DNA damage response (DDR) primarily by phosphorylating key DNA damage response proteins such as ATM kinase or apurinic/apyrimidinic endonuclease 1 (APE1), and suppression of this enzyme activity has been shown to regulate the DDR process and cancer progression as recently reviewed by multiple authors [72,73,74]. This evidence concerns the gene APEX1 and cancer.