– Apurinic-Apyrimidinic Endonuclease 1 (APE1) is required for efficient DNA base excision repair. When the DNA glycosylase remove the damaged bases, APE1 cleaves the AP site to allow resynthesis and ligation to complete repair. APE1 stimulates the DNA binding activity of many transcription factors, which participate in cancer promotion and progression. APE1 regulates the redox state of multiple transcription factors, such as c-Jun, c-Fos, NF-kB, p53. APEN is also involved in calcium-dependent down-regulation of PTH expression.