Cancer cell lines derived/transformed from mature B cells that have undergone BCR selection include multiple myeloma (n = 25), mantle cell lymphoma (n = 4), Burkitt lymphoma (n = 10) and chronic lymphocytic leukemia-small lymphocytic lymphoma (CLL, n = 4) etc. High expression of both IGH (heavy chain) and IGK/L (light chain) rearrangement are detected in all of the mantle cell lymphoma, Burkitt lymphoma and CLL-small lymphocytic lymphoma cell lines (Fig. 1). The gene discussed is BCR; the disease is B-cell chronic lymphocytic leukemia.