A t(14;18) chromosomal translocation is found in -85% of follicular lymphomas by both cytogenetic and molecular analyses. This rearrangement deregulates expression of the bcl-2 proto-oncogene by translocation into the immunoglobulin heavy chain locus and is probably mediated by illegitimate V(D)J recombination. We have developed a quantitative nested PCR method for detecting this event in lymphocytes of healthy individuals. Genomic DNA is purified from peripheral blood lymphocytes, and 2.5 ug

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... epresenting 4X10^ cells) are amplified with translocationspecific primers under conditions in which a single copy, if present, will give a detectable PCR product Multiple replicates are analyzed for each individual, and Poisson statistics are then used to estimate the translocation mutant frequency. We have examined lymphocyte DNA from 34 healthy individuals by this assay and found the frequency of cells with t(14,18) to range from <0.8-96Xl0-'. The molecular nature of the translocations has been investigated by determining the DNA sequence at the translocation junctions. In several individuals, multiple isolates of the same translocation event were recovered, indicating that the cell with the original translocation had undergone clonal expansion. In addition, multiple independent translocations were shown to occur within an individual. Since this translocation appears to be one step in the progression of a normal cell to a cancer cell, this assay may have utility as an effects biomarker for environmental carcinogen exposure.