The gene trap vector pUC17, which contains an En splice acceptor, lox71 site, lacZ gene, neo cassette, loxP site and lox2272 site, was inserted into intron 4. The insertion results in truncation after the first 320 bp of endogenous sequence and resulting in the loss of the catalytic domain. QRT-PCR confirmed the transcriptnis virtually absent in E11 embryos.