A targeting vector containing an ACN self-excising loxP-flanked neomycin cassette and a diptheria toxin (DTA) gene was used to replace the threonine residue at position 167 with an alanine residue. The left homology arm included all of exons 2, 3, and 4 (including the T167A substitution codon in exon 4), while the right homology arm contained part of intron 4.