Although the t(14;18) chromosome translocation has been demonstrated to be a highly consistent feature of follicular lymphomas, the underlying mechanism generating this fusion has remained uncertain. To examine this question further, a polymerase chain reaction strategy has been devised to permit the amplification and direct sequencing of the resultant 14q+ and 18q- reciprocal junctions. Direct sequence analysis of amplified 14q+ junctions established that 7 of 11 tumors contained a bcl-2 (mbr) sequence fused to an immunoglobulin JH region (five were J6 and two were J5). One of these junctions had an unusual configuration with the bcl-2 and JH sequences separated by a recognizable DH region. This finding suggests that at least some of the junctional sequences, previously thought of as N insertions, may be fragments of unrecognized DH regions. It was also possible to amplify and sequence 18q- junctions using a primer based on the DH recombination signal sequences. Several 18q- junctions were shown to consist of DH/bcl-2 (either mbr or mcr) fusions. In two tumors the 14q+ and 18q- junctions were fully sequenced, and it was demonstrated that the bcl-2 sequence was conserved during mbr and mcr translocations. This contrasts with previous analyses that demonstrated either loss or duplication of several bases at the breakpoints in the bcl-2 gene.

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