Exons 3 through 8 were replaced with the corresponding wild-type sequence with a C-terminal FLAG tag (6Y-FLAG), an FRT-flanked neo cassette, a loxP site and the corresponding sequence in which the six immunoreceptor tyrosine-based activation motif (ITAM) tyorsines mutated to encode phenylalanine followed by a C-terminal Myc tag (6F-Myc). Cre-mediated recombination removed the 6Y-FLAG tag and neo cassette. The 6F-Myc tag remains intact.