A gene trap consisting of a loxP site, an FRT flanked-cassette containing En2 spicing acceptor, IRES, lacZ, pA, hBactP, neomycin resistance gene, and pA, followed by exons 3 to 5, and a second loxP site was inserted after exon 2. The En2 SA/LacZ/neo cassette was removed via flp-mediated recombination. Exons 3-5 were deleted via cre-mediated recombination in the germline.