A single lox site was introduced 0.5-kb downstream of Exon 4. A reporter fusion construct containing a FRT site, splice acceptor (derived from 0.6kb upstream of exon 6), the cDNA from exons 6-18, IRES, b-galactosidase-neo, polyA, FRT site and an additional loxP site were inserted within intron 5 by homologous recombination.