A loxP site flanked puromycin resistance gene/transcription stop cassette was inserted into intron 1 and mutations were created in exon 4 altering codon 53 from phenylalanine to glutamine (p.F53Q) and codon 54 from phenylalanine to serine (p.F54S). The second transactivation domain is incapacitated once the stop sequences are removed by cre expression.