An FRT site flanked puromycin resistance gene cassette and a loxP site were inserted into intron 3 and a second loxP site, an F3 site flanked neomycin resistance gene cassette and an EGFP reporter gene cassette were inserted downstream of exon 7. The puro and neo cassettes were removed through subsequent flp-mediated recombination. Floxed exons 4-7 were deleted through subsequent cre-mediated recombination, leaving a reporter null-allele. RT-PCR experiments confirmed the absence of transcription from this allele.