The RAD4 gene of Saccharomyces cerevisiae is required for the incision step of excision repair. We have cloned the RAD4 gene and determined its nucleotide sequence. RAD4 encodes a somewhat basic protein of 754 amino acids (aa) with an Mr of 87,173. RAD4 contains several groups of 4-7 consecutive basic aa residues that could be involved in DNA binding and it also contains an alpha-helix-turn-alpha-helix motif for DNA binding. Like several other DNA repair proteins of S. cerevisiae, the C terminus of RAD4 protein is highly acidic.