The sequences coding for the three regulatory serines: Ser-112, Ser-136, and Ser-155 were all altered to code for alanines while an FRT flanked PGK-neomycin resistance cassette was inserted into the 3' UTR. The neomycin gene was subsequently excised by crossing mutant mice to transgenic mice that express FlpE recombinase. Western blot analysis demonstrated that the level of protein expression was similar to that of the wild-type protein but that the protein produced could no longer be phosphorylated at the three regulatory serines.