A splice acceptor-STOP/neomycin selection cassette flanked by loxP sites was introduced on the 5 end of exon 3, which was modified to introduce point mutations that generate a threonine to alanine substitution at amino acid 14 (T14A) and a tyrosine to phenylalanine substitution at amino acid 15 (Y15F). A14 and F15 cannot be phosphorylated and thus this mutant becomes non-inhibitable by phosphorylation. Cre-mediated recombination removes the loxP-flanked STOP cassette.