A loxP-flanked Neomycin selection cassette is inserted downstream of the gene, and a loxP-flanked hygromycin cassette is inserted upstream of the gene. Cre-mediated recombination removed the both floxed cassettes, the promoter and 81.8 kb of Firre sequence. No gene product is detected by in situ hybridization of whole mount E11.5 homozygous embryos. (J:261345)