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During recombination, heavy chain diversity gene segments can be rearranged in six possible reading frames (RF) to encode part of an antibody's antigen recognition site. In mice, RF1 is the preferred reading frame. A targeting vector was constructed to replace the heavy chain diversity segment DFL16.1 segment with a mutated DFL16.1 segment that shifted the bias from RF1 to RF2. This was done by inserting a thymidine nucleotide at the 5' terminus and a second thymidine nucleotide six nucleotides from the 3' terminus. The upstream insertion shifted the DFL start site from RF2 to a RF1. The downstream insertion shifted the Dh-Jh microhomology region from RF1 to RF2. The second mutation also shifted the distal encoded TAG termination codon from RF3 to RF1. A downstream loxP flanked neo cassette was included in the vector. This vector was used in targeting ES cells containing the Ightm1.1Lnit mutation that had the most distal heavy chain diversity segment, DQ52, replaced with a loxP site. ES cells containing the two mutations in cis were selected and chimeric mice generated. These mice were crossed with a cre transgenic mice to remove the downstream 11 diversity gene segments and the neo cassette. This genetic manipulation left the mutated DFL16.1 segment as the only heavy chain diversity segment that can be used during recombination. DNA sequencing confirmed correct targeting. (J:142068)