A targeting construct was designed to insert a loxP site in intron of the 13th coding exon. Coding exon 14 was fused to a cDNA encoding coding exons 15, 16, and 17 together with a polyadenylation signal after the stop codon. This was then followed by a neomycin resistance cassette and a second loxP site. An additional exon 14 sequence with a stop codon preceding the active site cysteine codon was inserted after the second loxP site. Immunoblot showed that the presence of the loxP sites and neo did not interefere with translation.