A Gly to Arg point mutation at position 717 in exon 12 and a new SacII site in exon 12 were introduced and a floxed neo cassette were inserted after exon 14 via homologous recombination. Cre mediated recombination removed the neo cassette. The point mutation eliminates the diphthamide modification on the protein but leaves the diphthamide biosynthesis machinery intact.