Class II apurinic or apyrimidinic endonucleases catalyze the hydrolysis of DNA backbone immediately 5_ of an apurinic/apyrimidinic residue by creating internal breaks, generating a single-strand break with 5'-deoxyribose phosphate and 3'-OH ends. Class II AP endonuclease play a crucial role in the base excision repair pathway. (Adapted from PMID:19401441).