Exon 3 was flanked by loxP sites and a loxP/FRT-flanked neomycin resistance gene followed by an IRES-lacZ cassette were inserted downstream of exon 3 via homologous recombination. The neomycin selection cassette was removed via Flp-mediated recombination and exon 3 was removed via cre-mediated recombination. Northern blot analysis confirmed the absence of RNA expression. (J:226682)