A loxP site was inserted upstream of exon 1 and a loxP-flanked neomycin and thymidine kinase selection cassette was inserted downstream of exon 2. Cre-mediated recombination removed the neo cassette. Exons 1 and 2 were subsequently removed from properly targeted mice by crossing to transgenic mice carrying Tg(EIIa-cre)C5379Lmgd. (J:94077)