Threonine 140 of Exon 3 was mutated to alanine (by introducing an A95G base pair change) and a loxP-flanked neomycin resistance cassette was placed in intron 2. A B6(Cg)-Tyrc-2J/J-derived embryonic stem cell line was used to create the mutation. The neomycin cassette was excised with Cre, leaving a single loxP site.