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Point mutations were introduced into the endogenous gene by homologous recombination. Codon 357 was converted from TAC to TTC, codon 390 from TAT to TTT and codon 442 from TAC to TTC. Each of these changes is predicted to alter a conserved tyrosine residue in the encoded protein to a phenylalanine residue. These amino acids are in the large intracellular loop of the protein. An frt-flanked neomycin selection cassette in intron 10 was removed via Flp-mediated recombination, leaving a single frt site in intron 10 in the final allele. Affinity purification and western blotting experiments demonstrated that the encoded protein is expressed at similar levels to controls, but does not cross-react to a phosphotyrosine-specific monoclonal antibody directed at residue 390. Similar results were seen in immunohistology experiments. (J:128926)