The rich history of binding studies of monoclonal Ig produced by PCs have revealed no universally common binding properties, but instead, groups of PCs with specific antigen-binding activities to haptens such as phosphorylcholine, dextrans, fructofuranans or dinitrophenyl. Subsequently, it was found that PCs with similar binding specificities not only expressed the same idiotype but rearranged the same VL and VH genes to express a characteristic monoclonal antibody. In a study of silicone-induced mouse plasmacytomas (SIPCs), we have found; i). antibodies secreted by SIPCs bind to different antigens in a manner similar to that observed for natural autoantibodies, ii). the expressed Ig heavy genes are restricted in V gene usage to the VH-J558 family and iii) secondary rearrangements occur at the light chain level with at least three SIPCs expressing both Vk and Vl light chain genes. These results suggest that SIPCs utilize a restricted population of B cells that may still be undergoing rearrangement, thereby bypassing the allelic exclusion normally associated with expression of antibody genes. Mechanistically, if the secondary rearrangement is not from the same allele, a possibility could exist in which two light chains (Igk:Igk or Igk:Igl) can be simultaneously expressed, thereby leading to an apparent violation of allelic exclusion. Dual expressing lymphocytes have been reported, but through the inefficient method of cell sorting. We have developed a technique of micro-manipulation of cells to determine whether two VL genes are expressed from a single lymphocyte. While these dual-expressing cells represent a minor subset (<10%) of the lymphocyte population, the continued presence of these cells in many PCs suggests an important role in the development of the B cell repertoire. Based on the fact that the Igk:Igl cells are Ly 1+, represent a small, but stable percentage of the tumor cell population, exhibit limited N-region sequences and are located in the peritoneum, we suggest that they may be a self-renewing long-lived B cell. Besides playing a major role in the transformation process through aberrant translocations that occur between IG and c-MYC, BCL1 or BCL2, IG rearrangement has typically been used as a prime indicator of the maturation status of a B cell tumor. Studies of V-gene usage and the presence of somatic mutational activity have also lead to the hypothesis that one or a few antigens could be a driving force behind the clonal evolution of the tumor. There has been considerable debate, however, as to whether some B cell tumors such as B-CLL, Marginal zone (MALT) lymphomas, BL, AIDS-NHL, Mantle cell lymphomas and DLCL present as mutated or non-mutated V-region lymphomas. While the VH regions have been thoroughly studied in B cell tumors in the anticipation that much of the binding site specificity will be dictated by VH, studies of the VL region have been less informative. We have now expanded our original VL studies in PCs to include a systematic study of VL usage in B-CLL and other human B lymphomas in an effort to determine; i). V-region usage ii).whether biallelic expression can be considered a stage of maturation and iii). the presence of somatic mutational activity. In order to more efficiently assess V-region usage from large resources of tumor samples, we are devoloping a fluorescence based four color assay of amplified V-region RT-PCR products using the Biorad "I-Cycler Real Time" PCR optical system. In a preliminary study of B-CLL, we have already determined that about 18% of the cases exhibit biallelic expression of VL regions (compared to 27% in the literature). We also find that Vk1 is the most frequently expressed light chain V region in B-CLL which is consistent with the fact that about 56% of expressed VL regions expressed in peripheral blood are Vk1. Ultimately, our goal is to define the role of the dual expressors in the progression of B cell malignancy and perhaps present the dual expressor as a potential therapeutic target.