A loxP site was inserted upstream of exon 1 and an FRT site flanked neomycin resistance gene cassette and a second loxP site into intron 1. mRNA expression levels were studied through RT-PCR and determined that the neo cassette interfered with the transcription of the allele. Therefore the neo cassette was removed through subsequent flp-mediated recombination. Exon 1 was deleted by subsequent cre-mediated recombination, creating a null allele. RT-PCR confirmed depletion of transcripts from this allele. (J:239934)