XRCC3 promotes homology-directed repair of DNA damage in mammalian cells. This study demonstrates that XRCC3 is essential for error-free homology-directed repair of DNA double-strand breaks (DSBs). Using a novel fluorescence-based assay, researchers found that XRCC3-deficient hamster cells showed a 25-fold decrease in homology-directed repair, which could be restored by expressing XRCC3. These results indicate that XRCC3 is critical for homologous recombination, which is vital for maintaining genomic integrity. The study also shows that XRCC3 interacts with Rad51, a key protein in homology-directed repair. The irs1SF cell line, which is defective in XRCC3, exhibits a defect in DSB-induced homologous recombination. However, this defect can be corrected by expressing XRCC3. The study further confirms that the fluorescent cells have a functional GFP gene arising from gene conversion, indicating that the repair process is effective. The results highlight the importance of XRCC3 in homologous recombination and its role in maintaining genomic stability in mammalian cells. The study also shows that other proteins, such as XRCC2, Rad51, and Rad52, do not complement the recombination defect in the irs1SF cells. The findings suggest that XRCC3 is a key factor in the repair of DNA damage and the maintenance of genomic integrity in mammalian cells.XRCC3 promotes homology-directed repair of DNA damage in mammalian cells. This study demonstrates that XRCC3 is essential for error-free homology-directed repair of DNA double-strand breaks (DSBs). Using a novel fluorescence-based assay, researchers found that XRCC3-deficient hamster cells showed a 25-fold decrease in homology-directed repair, which could be restored by expressing XRCC3. These results indicate that XRCC3 is critical for homologous recombination, which is vital for maintaining genomic integrity. The study also shows that XRCC3 interacts with Rad51, a key protein in homology-directed repair. The irs1SF cell line, which is defective in XRCC3, exhibits a defect in DSB-induced homologous recombination. However, this defect can be corrected by expressing XRCC3. The study further confirms that the fluorescent cells have a functional GFP gene arising from gene conversion, indicating that the repair process is effective. The results highlight the importance of XRCC3 in homologous recombination and its role in maintaining genomic stability in mammalian cells. The study also shows that other proteins, such as XRCC2, Rad51, and Rad52, do not complement the recombination defect in the irs1SF cells. The findings suggest that XRCC3 is a key factor in the repair of DNA damage and the maintenance of genomic integrity in mammalian cells.