A targeting vector was designed to insert a loxP site upstream of exon 1 and a second loxP site downstream of exon 1 followed by an FRT-flanked neomycin resistance (neo) cassette and a third loxP site downstream of exon 1. Cre-mediated recombination removed exon 1 and the neo cassette. Immunoprecipitation of cerebellum, adrenal medulla and cerebral cortex tissues confirmed that no protein was expressed in mutant mice.