CLONING HUMAN DNA-REPAIR GENES

Many human genes involved in the repair of UV damage have been cloned using different procedures and they have been of great value in assist ing the understanding of the mechanism of nucleotide excision-repair. Genes involved in repair of ionizing- radiation damage have proved mor e difficult to isolate. Positional cloning has localized the XRCC5 gen e to a small region of chromosome 2q33-35, and a series of yeast artif icial chromosomes covering this region have been isolated. Very recent work has shown that the XRCC5 gene encodes the 80 kDa subunit of the Ku DNA-binding protein. The Ku80 gene also maps to this region. Studie s with fission yeast have shown that radiation sensitivity can result not only from defective DNA repair but also from abnormal cell cycle c ontrol following DNA damage. Several genes involved in this 'checkpoin t' control in fission yeast have been isolated and characterized in de tail. It is likely that a similar checkpoint control mechanism exists in human cells.