A loxP site was inserted upstream of exon 1 and a FRT-flanked neomycin resistance gene and a second loxP site were inserted downstream of exon 1. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination excised exon 1. Real-time PCR confirmed complete loss of mRNA transcript and immunohistochemistry confirmed the absence of protein in brain. (J:250058)