BCR and B-cell chronic lymphocytic leukemia: Previously, to investigate the role of a monoclonal BCR in the progression of CLL in mice, we generated MD4+/−/Eμ-TCL1 mice by crossing Eμ-TCL1 mice, which spontaneously develop CLL with all the clinical features of aggressive human CLL [25, 26], with MD4+/− transgenic mice, which produce a monoclonal BCR against HEL [27–29].