Exon 1 was replaced with a version of exon 1 in which the asparagine at position 159 was deleted. A floxed cre cassette that was inserted downstream of exon 1 for selection purposes was removed by cre-mediated recombination using a transient cre expressing plasmid. The authors state that this mutation should produce a dominant negative protein. Expression of only the modified transcript was confirmed by RT-PCR analysis.