Our results indicate that there are two major breakpoint cluster regions in chromosome 18 DNA for t(14;18) translocations in follicular lymphomas. The absence of a pFL-1 homologous transcript in a cell line containing a pFL-2-detectable translocation suggests that there may be two different pathogenetic consequences of t(14;18) translocations. One possibility is that, despite the distances between them (greater than 20 kb), breakpoints in the two cluster regions in some way affect transcription of the same gene product, which has not yet been identified. Alternatively, two separate transcriptional units may be involved. The availability of DNA probes for each of the two t(14;18) breakpoint cluster regions will allow further studies regarding the biologic significance of these two genetically distinct classes of t(14;18) translocations.