Among these sixteen genes, two FA proteins, FANCI and FANCD2, form a stoichiometric complex called the ID complex. Eight FA proteins, FANCA. During the ICL repair processes, the ID and FA core complexes promptly accumulate at the ICL sites in chromosomes, and promote ICL repair with the other FA-related proteins. The FA core complex, which contains a ubiquitin E3 ligase BAY 73-4506 subunit, then monoubiquitinates both the FANCI and FANCD2 subunits of the ID complex. In particular, monoubiquitinated FANCD2 plays an essential role in the recruitment of downstream nucleases, which remove the bases with ICL. The ID complex preferentially binds to branched DNA in vitro, and the monoubiquitination of FANCD2 in the ID complex is robustly enhanced in the presence of DNA. FANCD2 is sitespecifically monoubiquitinated by FANCL, and cells with a mutation of the targeted FANCD2 Lys residue are remarkably defective in the ICL repair. This fact indicates that the FANCD2 monoubiquitination is essential for the ICL repair by the FA pathway. In addition, FANCD2 possesses histone chaperone activity, which modifies the chromatin structure by promoting histone deposition/eviction around the ICL sites. The chicken FANCD2 R305W mutant, in which Arg305 is replaced by Trp, is specifically defective in the histone chaperone activity in vitro and in vivo, and complements the ICL repair-defective phenotype of the FANCD22/2 DT40 cells with a significantly reduced rate. These results revealed that the histone chaperone activity of FANCD2 may play an important role during ICL repair, probably by modifying the chromatin structure to allow access for the proteins required for the downstream steps of the FA pathway. Importantly, the human FANCD2 R302W mutation, which corresponds to the chicken FANCD2 R305W mutation, has been identified as an FA causative mutation in a patient. A number of FANCD2 mutations, which are generally considered to reduce protein stability, have been identified in FA patients. However, the means by which the residual FANCD2 protein functions participate in the ICL repair remain poorly understood. In the present study, we focused on the chicken FANCD2 L234R mutation, which corresponds to the human FANCD2 missense mutation at the Leu231 residue, found in an FA patient. These in vivo results suggest that the FANCD22/2 DT40 cells with the cFANCD2 L234R mutant reflect the characteristics of the FA patient cells. We then purified the cFANCD2 L234R protein, and performed biochemical analyses. Interestingly, we found that cFANCD2 L234R is clearly defective in the ID complex formation with cFANCI. Intriguingly, a cell-based pull-down assay also revealed that cFANCD2 is defective in the cFANCI binding in vivo. Although the original report did not address the stability of the FANCD2 protein bearing this mutation, the level of the protein might be reduced because of the lack of the FANCI interaction. In the ID complex, both FANCD2 and FANCI are site-specifically monoubiquitinated by the FA core complex containing other essential FA proteins.