The targeting vector consists of a diphtheria toxin A-negative selection cassette (DTA) followed by exons 3 to 6 flanked by loxP sites and a neomycin-positive selection cassette (NeoR) flanked by frt sites downstream of exon 6 but before the loxP site. Cre-mediated recombination removed exons 3-6 and the NeoR cassette. (J:241919)