A floxed neomycin selection cassette was inserted upstream of the exon encoding the KIX domain and the KIX containing exon itself was mutated so that the tyrosine at amino acid residue 630 became alanine, the alanine at residue 634 became glutamine and the tyrosine at residue 638 became alanine (Y630A, A634Q and Y638A). Transient cre expression then excised the selection cassette. Expression and stability of the mutated product were normal.