Cellular DNA is continuously damaged by endogenous and exogenous reactive species among them oxygen radicals,alkylating agents,X-rays and environmental mutagens.The outcome of DNA damage is generally adverse,contributing to degenerative processes such as aging and cancer.DNA-repair pathways represent multiprotein systems catalyzing complex transformations of DNA.A central goal of molecular biology of DNA repair is to understand the molecular basis employed by repair protein machines to fight against genotoxic stress.Affinity labeling is regarded as advantageous tool for analysis of specific interactions of proteins and their ligands in multicomponent systems and in nuclear or cellular extracts.Photoaffinity labeling technique has been developed to study assembly of base excision DNA repair machines around DNA.The main idea of the approach consists in introduction of photoreactive moieties in the course of DNA transactions by the activity of DNA polymerases.