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.