In enzymology, DNA-(apurinic or apyrimidinic site) lyase, also referred to as DNA-(apurinic or apyrimidinic site) 5'-phosphomonoester-lyase (systematic name) or DNA AP lyase (EC 4.2.99.18) is a class of enzyme that catalyzes the chemical reaction of the cleavage of the C3'-O-P bond 3' from the apurinic or apyrimidinic site in DNA via beta-elimination reaction, leaving a 3'-terminal unsaturated sugar and a product with a terminal 5'-phosphate. In the 1970s, this class of enzyme was found to repair at apurinic or apyrimidinic DNA sites in E. coli and in mammalian cells. The major active enzymes of this class in bacteria, and specifically, E. coli is . This enzyme encompasses a family of lyases that cleave carbon-oxygen bonds.