The construct includes an FRT-flanked PGK-neomycin selection cassette upstream of exon 1 followed by a 2.1 kb region encompassing the predicted promoter region upstream of exon 1 and 160 bp of intron 1 flanked by loxP sites. The selection cassette was subsequently excised by FLP recombinase. Cre mediated recombination removed exon 1. Northern blot analysis confirmed loss of all RNA isoforms.