Three mice showed signs of monoclonal B-cell lymphocytosis (MBL)—a less progressed form of CLL—in which the percentage of B-1a cells (CD5+B220+CD19+) in the blood is between 2% and 20%, with simultaneous expansion in the spleen and bone marrow (Fig. 2C and data not shown). Here, CD19 is linked to B-cell chronic lymphocytic leukemia.