An FRT-flanked cassette with a splice acceptor, lacZ gene, neomycin resistance gene and polyadenylation sequence (en2SA-LacZ-neo-pA) followed by a 3' loxP site was inserted upstream of the second coding exon. An additional loxP site was inserted downstream of the second coding exon. Flp-mediated recombination removed the lacZ/neomycin resistance cassette and left the second coding exon floxed.