Mutant rodent cell lines with hypersensitivity to DNA damage resulting from a defect in cellular response to the damage have contributed to many of the recent advances in our knowledge of DNA-repair processes. Many of these mutants have been classified by genetic complementation analysis. They have proved to be excellent recipients of human DNA in transfection experiments, and from those transfectants with restored resistance to DNA damage it has been possible to isolate the foreign DNA responsible for correcting the defect. Several human genes coding for DNA-repair proteins have now been cloned. Until recently, there was apparently no correlation at the genetic level between the artificially produced rodent mutants and the inherited human diseases associated with sensitivity to DNA-damaging agents, but several of the human genes cloned in rodent mutants now tum out to correct defects in cells representing the human diseases, too. Others are homologous to yeast DNA-repair genes, and it is clear that at least some of the proteins involved in DNA repair are highly conserved through evolution. Mutants have also provided the material for comparative biochemical studies of DNA repair, and we are nearer to understanding the complexities of this process. The tables presented here list rodent mutant cell lines sensitive to ultraviolet light, to ionizing radiation and to cross-linking agents. These 3 categories of mutants tend to be the best characterized, both biochemically and genetically, and while other mutants (selected on the basis of sensitivity to other DNA-damaging agents) do exist, they are less clearly related to the known pathways of DNA repair. In some cases, however, cross-sensitivities mean that such mutants may fall into one of the main categories. For example, mutants sensitive to alkylation damage may also be sensitive to ionizing radiation or to UV, pointing to a convergence of different pathways, with common enzymes at certain steps. When sensitivity is described as ''extreme'', the change in D10 is 10-fold or more. ''Moderate'' sensitivity applies to a D10 decrease of between 2- and 10-fold, and ''slight'' indicates up to a 2-fold decrease in D10. It is worth pointing out that many of the enzymes thought to be involved in DNA repair are still not represented among these mutants. Most UV-sensitive cells, for instance, are defective in incision, and there appears to be a wealth of proteins involved at or before this step. No mutants have been isolated with identified defects at later steps such as excision or ligation. Incision-competent mutants may instead be deficient in a process other than excision repair that deals with damage, such as post-replication recovery. Presumably the steps that are not represented in mutants make use of enzymes that are required for other, essential cellular reactions, so that mutants, whenever and however they arise, will be non-viable. ''Leaky'' mutants, typically of relatively modest sensitivity (and therefore rather difficult to identify), may represent partial defects in essential genes. Temperature-sensitive mutants, in which a lethal defect is revealed at high temperature but cells are viable if maintained at a lower temperature, may be another route to follow in the search for the missing genes.
