The recombinase site flanked reporter cassette (FRT site, engrail2 splice acceptor, IRES, LacZ, polyadenylation sequence and loxP site) and selection cassettes (beta-geo, polyadenylation site, FRT site and loxP site) were inserted upstream of exon 5. An additional loxP site was inserted downstream of exon 6. Flp mediated recombination removed the reporter and selection cassettes. Cre-mediated recombination removed exons 5 and 6, which encode the catalytic domain.