An IRES, lacZ and floxed neomycin resistance cassette were inserted into exon 7 via homologous recombination resulting in disruption of the DNA binding domain. Cre-mediated recombination removed the floxed neo cassette. The mutant transcript encodes the first 6 exons of the gene followed by the IRES-lacZ message. The pattern of lacZ expression in mutant animals is similar to that of the endogenous protein.