The use of Viral probes to study DNA repair processes in mammalian cells has demonstrated its prodigious advantages. Expressing cellular repair functions in viral DNA greatly simplifies the strategy of investigation. The cellular constitutive repair activities have been probed as host cell reactivation and recombinational multiplicity infection reactivation by double-stranded DNA viruses such as Herpes Simplex Virus, adenovirus and SV 40. Single-stranded parvoviruses have been employed successfully to probe inducible cellular repair or recovery processes. It has been shown that an error-prone recovery activity resembling "SOS" response in E. coli may be induced in mammalian cells by a series of DNA-damaging agents. The inducing signal resides in DNA damage itself. Some DNA damages such as apurinic or apyrimidinic site may serve as the proper substrate of the induced repair function.