UV light-induced mutagenesis in bacteria is a genetically controlled process dependent on induction of some cellular functions, provoked initially by unrepaired photolesions in the DNA. Experiments on the extent and fidelity of in vitro DNA systhesis on UV-irradiated templates by bacterial mammalian DNA polymerases suggest a crucial role for 3' to 5' exonuclease (proofreading) activity in UV light-induced mutagenesis. Two-stage carcinogenesis (initiation and promotion) is discussed in terms of two-stage mutagenesis (mutation fixation in the DNA and mutation expression). A unifying concept for both mutational and viral malignant transformation is proposed.