Until recently, investigators interested in analyzing the repair of chromosomal double-strand breaks (DSBs) in mammalian cells have been limited by the inability to introduce defined DSBs within the genome. Traditional methods of introducing breaks have included irradiation or the introduction of restriction enzymes (1 ;seeChapter 38 ); however, both of these methods cause multiple lesions at different chromosomal loci. Many of these types of studies have relied on cytogenetics for the detection of gross genomic changes owing to misrepair at these damaged sites.
