Exons 1-13 along with a 2-kb upstream region were replaced with a R4 integrase attB- and attP- flanked neomycin cassette. The neomycin cassette was then removed by transient expression of R4 integrase in the targeted clones. RT-PCR analysis confirmed absence of mRNA expression in E10.5 homozygous mutant embryos and yolk-sac tissues. (J:215242)