A loxP site was inserted 371 bp upstream of exon 3, and an FRT site, loxP site, neomycin resistance gene, another FRT site and a third loxP site were inserted 351 bp downstream of exon 5. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination deleted exons 3-5. Excision of these exons eliminates the part of the enzyme containing the catalytic residues 107HPGH110 encoded in exon 3 and introduces a stop codon following exons 6 and 7. RT-qPCR confirmed absence of transcript and immunofluorescence of skin and tarsal plate shows absence of protein.