An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted upstream of exon 14. An additional loxP site was inserted downstream of exon 14. Flp-mediated recombination removed the selection cassette and left exon 14 floxed. Cre-mediated deletion of exon 14 produces a protein that contains the first 701 residues and 25 additional artificial residues.