Fanconi anemia is a hereditary cancer susceptibility disorder characterized at the cellular level by spontaneous chromosomal instability and specific hypersensitivity to DNA cross-linking agents such as mitomycin C. This phenotype suggests a possible role for the Fanconi anemia proteins in the repair of DNA lesions induced by these agents, but the molecular mechanism underlying the defect in this disorder has not yet been identified. Here, we show that amongst eight so far identified complementation groups of Fanconi anemia, only fibroblasts derived from group D1 are defective in the formation of nuclear Rad51 foci after X-ray irradiation or mitomycin C treatment. This indicates that the FANCD1 gene product is uniquely involved in the assembly and/or stabilization of the Rad51 complex. Since DNA damage-induced Rad51 nuclear foci are thought to reflect repair of DNA double-strand breaks by homologous recombination, our results suggest that FANCD1 is likely to be involved in homologous recombination-dependent repair.