A secretory trap vector possessing a splice acceptor followed by a linker protein, a transmembrane domain, beta-geo, an IRES domain, and a placental alkaline phosphatase reporter gene was targeted to intron 2 of the gene locus. As a result of integration, exons 1 and 2 splice to the vector sequences resulting in a mutant fusion protein with no biological activity. Northern blots of RNA from E10.5 embryos indicated no wild-type transcript while RT-PCR indicated very low levels (under 3%) of wild-type transcript.