A floxed transcription disrupter cassette was inserted into intron 1, 137 bp upstream of exon 2 via homologous recombination. The transcription disrupter cassette includes an engrailed 2 splice acceptor, SV40 enhancer, Neo, and a Myc-associated zinc finger protein-binding site. RT-PCR analysis confirmed the absence of full length transcript and the presence of a mutant mRNA.