Cellular genomes are continually subjected to endogenous and environmentally induced structural alterations. Our environment contains a multitude of substances that are carcinogenic and which, in many cases, are thought to act via direct damage to DNA. In order to better understand the consequences of DNA damage in cell killing and carcinogenesis, it will be important to: (a) develop new and sensitive techniques for the identification of specific types of DNA lesions; (b) examine what influence the function or activity of a DNA sequence has on the distribution of DNA damage within that DNA sequence and on its repair efficiency; (c) examine what influence the cell cycle has on the efficiency of DNA repair.