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Eleven suppressors of the radiation sensitivity of Saccharomyces cerevisiae diploids lacking the Srs2 helicase were analyzed and found to contain codominant mutations in the RAD51 gene known to be involved in recombinational repair and in genetic recombination. These mutant alleles confer an almost complete block in recombinational repair, as does deletion of RAD51, but heterozygous mutant alleles suppress the defects of srs2::LEU2 cells and are semidominant in Srs2+ cells. The results of this study are interpreted to mean that wild-type Rad51 protein binds to single-stranded DNA and that the semidominant mutations do not prevent this binding. The cloning and sequencing of RAD51 indicated that the gene encodes a predicted 400-amino-acid protein with a molecular mass of 43 kDa. Sequence comparisons revealed homologies to domains of Escherichia coli RecA protein predicted to be involved in DNA binding, ATP binding, and ATP hydrolysis. The expression of RAD51, measured with a RAD51-lacZ gene fusion, was found to be UV- and gamma-ray-inducible, with dose-dependent responses.