A FRT site flanked targeting vector containing PGK-Neo selection cassette was utilized in the construction of this mutant. This selection cassette and a loxP site was inserted upstream of exon 2. A second loxP site was inserted downstream of exon 3. Mutant mice were bred with Gt(ROSA)26Sortm1(FLP1)Dym mice (on a C57BL/6 congenic background (N10)) to remove the neo selection cassette.