Endonuclease IV (Tth) is a thermostable apurinic-apyrimidinic (AP) class II DNA endonuclease from Thermus thermophilus. This enzyme is involved in the DNA base excision repair (BER) pathway, which catalyzes the cleavage of the DNA phosphodiester backbone at AP sites via hydrolysis, leaving a 1 nucleotide gap with 3′-hydroxyl and 5′ deoxyribose phosphate (dRP) termini. AP sites are locations in DNA that do not contain a purine or pyrimidine base that arise spontaneously or as a result of DNA damage. The enzyme also exhibits 3′-diesterase activity.