Replacement of sequence encoding exon 3 of the Xrcc4 gene with a loxP-flanked neomycin cassette. This cassette was then removed by crossing mice with germline-Cre expressing mice to generate this allele containing a deletion of exon 3. Any potential transcript from this allele that spliced from exon 2 to exon 4 or 5 would harbor a frameshift mutation. (J:65218)