Tyrosine residues at positions 175, 195 and 235 (p.Y175F, p.Y195F, p.Y235F), in exons 9-11, respectively, were mutated to phenylalanine residues. Western blot shows that the amount of protein in mutant thymocytes is similar to that of wild-type. A floxed neo was inserted between exons 9 and 10 and subsequently removed by crossing with deleter mice.