The majority of non-Hodgkin's B-cell lymphomas contain a t(14;18) translocation that places the bc12 gene into juxtaposition with the transcriptically active Ig heavy-chain locus, thus deregulating the expression of this proto-oncogene. The bc12 gene product is a membrane- associated mitochondrial protein that regulates cell survival through unknown mechanisms. Although overproduction of the normal protein appears sufficient for conferring a selective growth or survival advantage to B cells, point mutations that alter the coding region of translocated bc12 genes have been described previously by others in a lymphoma cell line. However, it is not known whether somatic mutations that alter BCL2 proteins occur in vivo or whether they result from chemotherapy or arise through other mechanisms. For these reasons, we obtained DNA from the t(14;18)-containing tumors of five patients who had not undergone treatment for their disease, and used a polymerase chain reaction (PCR)-mismatch technique for rapid identification of point mutations in a portion of the bc12 open reading frame (ORF) corresponding to the first 131 aminoacids (aa) of the 239 aa p26 BCL2 protein. DNAs from two t(14;18)-containing cell lines were also analyzed. Point mutations in this region of the bc12 gene ORF were detected in three of five patients' tumors and in both cell lines. PCR- mismatch analysis of bc12 in cell lines and non-Hodgkin's lymphoma cases that lacked the t(14;18) translocation was negative, thus establishing the specificity of these results. DNA sequencing determined that these mutations are predicted to produce aa substitutions in the BCL2 proteins of two of the primary tumors and one of the cell lines. Interestingly, two of the patients contained an identical C----T transition that resulted in a nonconservative aa substitution (proline----serine) at position 59 of the BCL2 protein. Further analysis excluded the possibility that these mutations represented hereditary polymorphisms or PCR artifacts. A cluster of four point mutations within the translocation + bc12 allele of one patient had hallmarks of the somatic hypermutation mechanism that is associated with Ig genes and that contributes to antibody diversity. Because of the region of the bcl2 gene analyzed in these t(14;18) translocations is located nearly 300 kbp from the Ig heavy-chain locus, our data suggest that the Ig gene somatic hypermutation mechanism can act over extreme distances of DNA. It remains to be established whether these somatic mutations that alter BCL2 proteins influence the pathobiology of nonHodgkin's lymphomas.