Looping out and deletion mechanism for the immunoglobulin heavy-chain class switch.

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Abstract
In the mouse pre-B-cell line 18-81, cells can switch production in vitro from immunoglobulin .mu. chain to .gamma.2b chain. The gene encoding the .gamma.2b chain is created by a rearrangement of the .mu. gene. This rearrangement always takes place within a homolog. In cells with a .gamma.2b gene, most of the time the gene segment encoding the constant region of the .mu. chain is deleted, but often the rearrangement leads to cells that produce no immunoglobulin, and all DNA sequences are retained. The latter result is due to an inversion. Inversions exclude the unequal sister chromatid exchange model of the heavy-chain class switch. Looping out is an intermediate step in the process of generating an inversion. Our findings demonstrate that the switch rearrangement occurs by looping out and deletion.