Multiple genes (many of which are designated RAD (for radiation resistance)) are required for cellular responses to DNA damage in the yeast Saccharomyces cerevisiae. In recent years a number of these genes have been cloned and sequenced and in some cases their polypeptide products have been purified and characterized biochemically. These studies are beginning to yield clues about the possible nature of the multiple biochemical pathways for DNA-damage processing in yeast.