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A 3.7-kb region (including the most proximal D-heavy gene segment, DQ52, all the J-heavy gene segments, and the Emu enhancer element (Rr150)) was replaced with a loxP-flanked neomycin resistance gene cassette and a fully assembled V-heavy gene segment. The V-heavy gene segment and about 2 kb of upstream sequence were isolated from the B1-8 hybridoma, which produces a 4-hydroxy-3-nitrophenyl acetyl-binding antibody. Founder mice were crossed with EIIA-cre transgenic mice to remove the neomycin cassette from the germline. Correct targeting was confirmed by Southern blot and genomic PCR. Homozygous mice had B cells expressing the knocked in immunoglobulin heavy chain. (J:144033)