Three serine phosphorylation sites (Ser 422/423/425) in the C-terminus of the gene were mutated to alanine to eliminate phosphorylation and a floxed neo cassette was inserted via homologous recombination. The mutations were S422A (TCC to GCC), S423A (AGT to GCT), and S425A (TCT to GCT). (J:309120)