An frt flanked neo cassette with a 3' loxP site was inserted downstream of exon 4. An additional loxP site was inserted downstream of exon 5 followed by a modified exon 5 in which all 6 intracellular tyrosines are replaced with phenylalanine and the last valine is deleted. Cre mediated removal of the wild-type exon 5 is needed to allow for expression of the modified exon 5. (J:150043)