A targeting vector was designed to insert a loxP site upstream of exon 1, cDNA encoding the sequence of exon 3 fused in frame to the endogenous exon 2, followed by an SV40 polyadenylation sequence bound to an FRT-flanked neomycin resistance (neo) cassette and a second loxP site. Duplicate exons 1-2 were inserted downstream of the second loxP site and upstream of the endogenous exon 3. The second exon 1 contains a mutation in amino acid 95 which alters the nucleotide sequence from CCG to CAC, resulting in an amino acid change from Proline to Histidine. Flp-mediated recombination removed the FRT-flanked neo cassette. (J:221404)