Exon 20 was replaced with a modified one in which nucleotide substitutions (AAG GCC CTG to GAG GCC CTT) result in the amino acid substitution of glutamic acid for lysine at position 923 (K923E) and introduced a BspTI restriction enzyme site. A floxed neo cassette inserted downstream of exon 21 was removed by cre-mediated recombination. In vitro kinase activity confirmed the absence of activity.