This gene trap vector includes a 5' viral long-terminal repeat, splice acceptor, neomycin resistance cassette, poly adenylation sequence, PGK promoter, Bruton tyrosine kinase coding sequence, splice donor and 3' viral long-terminal repeat. The vector inserted between exons 1 and 2 of the endogenous gene. Western blot analysis indicates an approximately 50% reduction of protein in heterozygotes. (J:204809)