Abstract: In the yeast Saccharomyces cerevisiae, at least ten genes are involved in excision repair of DNA damaged by UV radiation and by other agents that distort the DNA helix. Mutations in the RAD1, RAD2, RAD3, RAD4 and RAD10 genes render cells highly defective in the incision of damaged DNA, whereas mutations in the RAD7, RAD14, RAD16, RAD23 and MMS19 genes reduce the level of damage excision. This review summarizes the evidence for the involvement of these genes in excision repair and highlights the important features in the structures of the proteins encoded by the various RAD genes. The RAD3 protein has been purified and characterized in our laboratory, and it possesses single stranded DNA dependent ATPase and DNA helicase activities. The RAD3 helicase moves along the single-stranded DNA in the 5'----3' direction. We suggest that this activity plays a role in strand displacement synthesis during excision repair and in DNA replication.