A loxP site was inserted between exons 1b and 1c and a floxed neomycin resistance cassette was inserted in intron 2. Mice carrying the floxed neomycin resistance cassette were bred to EIIa-cre mice, resulting in removal of the cassette with lox P sites remaining between exons 1b and 1c and in intron 2. (J:98799)