DNA repair processes are critically important in the prevention of carcinogenesis, and currently much research is directed toward elucidation of the biochemical mechanisms by which DNA repair occurs. Techniques have been developed for examining individual genes to quantitate the lesions induced by various chemotherapeutic agents and to measure the rate of gene-specific DNA repair. In mammalian cells, the DNA repair response exhibits intragenomic heterogeneity-active genes are preferentially repaired and the transcribed strand of DNA is repaired more rapidly than the nontranscribed strand. These studies have provided new insight into the molecular biology of DNA repair and a new perspective on the role of repair processes in cellular resistance to DNA damage and malignancy. Elucidation of the mechanisms by which gene-specific lesions are formed and repaired will be important if we are to understand the fundamental processes of malignancy.