The following were inserted into intron 5 (after the second coding exon): an FRT site, En2 splice acceptor sequence, an IRES, the lacZ beta-galactosidase gene, poly(A) signal sequence, a loxP site, a neomycin resistance gene cassette, a second FRT site and a second loxP site. A third loxP site was inserted downstream of the gene after exon 6. This creates a knockout allele. (J:331393)