T4 PDG (pyrimidine dimer glycosylase) has both DNA glycosylase and AP lyase activity. The 16 kd protein recognizes cis-syn-cyclobutane pyrimidine dimers caused by UV irradiation. The enzyme cleaves the glycosyl bond of the 5' end of the pyrimidine dimer and the endonucleolytic activity cleaves the phosphodiester bond at the AP site.