A loxP site was inserted upstream of exon 2 and a loxP-flanked neomycin and thymidine kinase selection cassette was inserted downstream of exon 5. Cre-mediated recombination removed the neomycin cassette. Exons 2 through 5 were subsequently removed from properly targeted mice by crossing to transgenic mice carrying Hprttm1(CMV-cre)Brd. (J:94077)