We have investigated the expression of RAD4 gene, which is absolutely required for nucleotide excision repair of damaged DNA in Saccharomyces cerevisiae. Analysis of RAD4 mRNA by Northern blot and by S1 nuclease mapping indicated that the transcript was approximately 2.3 kb and did not contain an intervening sequence. The transcription of RAD4 gene appeared to initiate at 48 base pair upstream from the first ATG codon, which was determined by S1 nuclease mapping. The 89 KDa Rad4 protein was detected in Escherichia coli by SDS-polyacylamide gel electrophoresis, which was consistent with the expected size of 84.3 kDa Rad4 protein. The overexpressed Rad4 protein showed a toxicity in E. coli host, suggesting the inhibition of protein synthesis and/or degradation of host protein. Various fusion vectors between the RAD4 gene and lac’Z gene of E. coli were constructed and used to obtain purified fusion protein. We have also investigated with these fusion genes that which part of RAD4 gene shows toxicity in E. coli by dot blot analysis and western bolot analysis. The results obtained have indicated that the carboxyl-terminal part of Rad4 protein may be important for both the toxic effect in E. coli and for its role in DNA repair in S. cerevisiae.