The RAD4 gene of Saccharomyces cerevisiae is absolutely required for the incision step of UV-induced nucleotide excision repair. The previsously cloned RAD4 gene by phenotypic complementation of UV-sensitive rad4-4 mutants with a yeast genomic library (pPC1) appeared to localize in the right arm of chromosome V as determined by rapid chromosome mapping This cloned RAD4 gene did not contain a suppressor gene as determined by suppression test of various auxotrophic mutants containing nonsense mutations. In addition, all different plasmids harboring the cloned yeast DNA inserts conferred UV resistance to rad 4-4 mutants to a similar level regardless of their copy number. These results indicated that the pPC1 is a truly functional RAD4 gene and its complementing activity must be resulted from the yeast RAD4 gene.