Apurinic⁄apyrimidinic endonuclease (APE), an essential DNA repair enzyme, initiates the base excision repair pathway by creating a nick 5¢ to an abasic site in double-stranded DNA. Although the Chinese hamster ovary cells remain an important model for DNA repair studies, the Chinese hamster APE (chAPE1) has not been studied in vitro in respect to its kinetic characteristics.