Class switching from γ3 to γ2b production at pre-B cell stage was observed in an Abelson virus-transformed murine immature cell line that is able to perform VH to DJH recombinations followed by class switch recombinations. A series of Southern blotting experiments indicates that the class switching from γ3 to γ2b was mediated by the deletion mechanism of the intervening CH genes on the expressed chromosome and that the break points in the switch recombinations might be different from each other among six independent switching events from γ3 to γ2b. This in vitro switching system should provide us with much information on the mechanisms of class switch recombination.
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