A targeting vector was designed to insert a loxP site upstream of exon 4 followed by a frt-flanked neomycin resistance (neo) cassette and a second loxP site downstream of exon 8 via homologous recombination. Exons 4-8 contain two immunoreceptor tyrosine-based inhibition motifs (ITIM)that interact with the tyrosine phosphatases SHP1, SHP2 and SRC kinase. (J:180203)