Homologous recombination is a double-strand break repair pathway required for resistance to DNA damage and maintaining genomic integrity. In mitotically dividing vertebrate cells, the primary proteins involved in homologous recombination repair are RAD51 and the five RAD51 paralogs, RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3. In the absence of Rad51d, human and mouse cells fail to proliferate, and mice defective for Rad51d die before birth, likely as a result of genomic instability and p53 activation. Here, we report that a p53 deletion is sufficient to extend the life span of Rad51d-deficient embryos by up to 6 days and rescue the cell lethal phenotype. The Rad51dembryo-derived fibroblasts were sensitive to DNA-damaging agents, particularly interstrand cross-links, and exhibited extensive chromosome instability including aneuploidy, chromosome fragments, deletions, and complex rearrangements. Additionally, loss of Rad51d resulted in increased centrosome fragmentation and reduced levels of radiation-induced RAD51-focus formation. Spontaneous frequencies of sister chromatid exchange were not affected by the absence of Rad51d, but sister chromatid exchange frequencies did fail to be induced upon challenge with the DNA cross-linking agent mitomycin C. These findings support a crucial role for mammalian RAD51D in normal development, recombination, and maintaining mammalian genome stability. (Cancer Res 2005; 65(6): 2089-96)