A loxP site and an FRT-flanked neomycin selection cassette was inserted into intron 2. A single loxP site was inserted into intron 4. Exons 3 and 4 were deleted via Cre-mediated recombination of loxP sites. No detectable wild-type transcript or encoded protein is present in MEFs derived from homozygous mutant mice. (J:221224)