The mechanism of induced mutagenesis has been particularly well analyzed in E. coli after UV-irradiation. UV-mutagenesis appears to be an inducible phenomenon which requires the specific cleavage of the lexA gene product by the protease activity of RecA protein and the presence of the umuC gene product. Similar genetic requirements are necessary to detect mutagenesis of UV-irradiated bacteriophages (1, 2, see 3, 4, 5 for review). UV-irradiated λ phage has a better survival in UV-irradiated host cell than in unirradiated one. This UV-reactivation is only associated with mutagenesis of the damaged DNA phage (6). The most likely explanation for UV-mutagenesis mechanism is the induction of some type of DNA replication past pyrimidine dimers which will produce mutations on newly synthesized strand opposite to the lesions on the template strand (7). However, mutations in phage DNA appear to be produced not only in phage containing DNA damages but also in undamaged phage providing that the host cell has been UV-irradiated before infection. This result suggests that inducible host functions might decrease the fidelity of replication even on undamaged template. The exact mechanism of this untargeted mutagenesis is not yet understood. As in bacteria, UV-irradiation of mammalian cells gives rise to mutations.Studies with normal and classical xeroderma pigmentosum human cell lines indicate that excision repair synthesis is an accurate repair process (8). In consequence, mutations seem to be due to the presence of unrepaired UV-damages on DNA, probably pyrimidine dimers. On the other hand, xeroderma pigmentosum variant cells are more mutable by UV-light than normal cells and seem to be deficient in an error-free post-replication repair pathway (9). Experiments carried out in mammalian cells in order to detect any inducible error-prone pathway have been negative so far. For example, no mutagenesis increase has been observed in split dose experiments (10). In contrast, experiments using animal viruses as a molecular probe, in the same way as bacteriophages, have shown that induced mutagenesis could in fact be detected in mammalian cells.KeywordsSimian VirusPyrimidine DimerMonkey CellMonkey Kidney CellLytic CycleThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.