Yeast RAD54 gene, a member of the RAD52 epistasis group, plays an important role in homologous recombination and DNA double strand break repair. Rad54 belongs to the Snf2/Swi2 protein family, and it possesses a robust DNA-dependent ATPase activity, uses free energy from ATP hydrolysis to supercoil DNA, and cooperates with the Rad51 recombinase in DNA joint formation. There are two RAD54-homologous genes in human cells, hRAD54 and RAD54B. Mutations in these human genes have been found in tumors. These tumor-associated mutations map to conserved regions of the hRad54 and hRad54B proteins. Here we introduced the equivalent mutations into the Saccharomyces cerevisiae RAD54 gene in an effort to examine the functional consequences of these gene changes. One mutant, rad54 G484R, showed sensitivity to DNA-damaging agents and reduced homologous recombination rates, indicating a loss of function. Even though the purified rad54 G484R mutant protein retained the ability to bind DNA and interact with Rad51, it was nearly devoid of ATPase activity and was similarly defective in DNA supercoiling and D-loop formation. Two other mutants, rad54 N616S and rad54 D442Y, were not sensitive to genotoxic agents and behaved like the wild type allele in homologous recombination assays. Consistent with the mild phenotype associated with the rad54 N616S allele, its encoded protein was similar to wild type Rad54 protein in biochemical attributes. Because dysfunctional homologous recombination gives rise to genome instability, our results are consistent with the premise that tumor-associated mutations in hRad54 and Rad54B could contribute to the tumor phenotype or enhance the genome instability seen in tumor cells.In Saccharomyces cerevisiae, homologous recombination (HR) 1 represents an important means for the repair of DNA double strand breaks and other types of DNA damage and also for restarting stalled replication forks (1-5). HR depends on the use of a homologous DNA molecule as a template to eliminate double strand breaks and other lesions mostly with high fidelity. Impaired HR results in genetic instability and sensitivity to genotoxic agents (6) and can lead to cancer in humans (7). Double strand break repair by HR is dependent on genes of the RAD52 epistasis group, RAD50, RAD51, RAD54, RAD55, RAD57, RAD59, RDH54/TID1, MRE11, and XRS2, in which structure and function have been highly conserved (6). Before repair can occur, the ends of double strand breaks are processed by nucleolytic resection of the 5Ј ends of the breaks to yield ssDNA, which is then bound by Rad51 to form a helical nucleoprotein filament often referred to as the presynaptic filament. Nucleation of Rad51 onto the ssDNA is facilitated by recombination mediators, including Rad52, the Rad55-Rad57 complex, and Rad54 (6,8). Once assembled, the presynaptic filament mediates a search for homology in the homologous dsDNA partner and forms a DNA joint molecule (called D-loop) with the latter. The length of the nascent DNA joint molecule is extended by DNA branch migration (9...