A neomycin resistance cassette was inserted into exon 3, in the region encoding the kinase domain of the protein. RT-PCR analysis detects no transcript from the disrupted gene in thymocyte RNA from homozygous mutant mice. Immunoblot analysis detects no protein product from the targeted gene in thymocyte lysates from homozygous mutant mice and a reduced amount in lysates from heterozygous mutants.