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Exon 4 was disrupted by homologous recombination of a targeting vector containing stop codons in all three reading frames and a neomycin selection cassette that replaces amino acids 54 through 59. Transcript was undetected by RT-PCR analysis of activated lymphocytes obtained from homozygous mutant mice. ELISA indicated the absence of protein in splenocyte cultures derived from homozygotes. (J:82881)