A loxP site was introduced upstream of exon 1 and a floxed neo cassette was inserted downstream of exon 2 via homologous recombination. The floxed neo cassette was subsequently removed by mating mutant animals to cre deleter strain TgN(EIIa-cre)C5379Lmgd, resulting in animals carrying floxed exon 1-2 alleles.