Exon 15 was targeted to change leucine codon 2431 (CTG) to proline (CCA) (p.L2431P). The loxP site flanked neomycin resistance gene cassette that was inserted into intron 15 was removed through subsequent cre-mediated recombination. The mutation mimics the p.L2510P mutation found in Fanconi anemia (FA) subtype D1 (FANCD1) patients.