A loxP site was inserted upstream of exon 7. An FRT- and loxP-flanked neomycin resistance cassette were inserted downstream of exon 8. Flp-mediated recombination removed the selection cassette and left exons 7 and 8 floxed. Cre-mediated recombination removed exons 7 and 8. This mutation removes a portion of the Tyr-rich domain.