A loxP site, a neomycin resistance gene cassette and a second loxP site were inserted upstream of the alternative P2 promoter in intron 1. A third loxP site was inserted downstream of the promoter. This creates a conditional-ready isoform specific knock-out allele which, upon cre-mediated recombination, would abolish transcription of the betaA, betaB and betaC splice variants. (J:239886)