Publisher Summary This chapter discusses the use of DNA fragments of defined sequence for the study of DNA damage and repair. A wide variety of physical and chemical agents damages the DNA. The biological effects of such agents include cell death, neoplastic transformation, and mutations. Cells are normally capable of repairing some types of DNA damage by enzymatic modification of the DNA. In most cases, the details of the repair pathways will determine the ultimate biological effect of the initial lesions. The chapter applies some of the remarkable progress that has been made in nucleic acid technology to such problems. The basic approach involves the use of a DNA fragment of defined sequence as a probe to study the damage and repair of DNA. The advantage of using such a sequence is that the precise site of damage and the subsequent fate of the modified DNA can be determined rapidly. The chapter also overviews some of the examples related to DNA DNA sequence for studies of DNA damage and repair.