These considerations are in line with the results of in vivo labeling experiments that involve incorporation of nonradioactive deterium into newly synthesized DNA of dividing cells. These studies have shown B-CLL clones to be dynamic, having measurable birth rates from about 0.1 to>1.0 percent of the clone/day. Although only a minority of cells in a B-CLL clone can be shown to divide using this approach, estimates of the leukemic cell burden of typical B-CLL patients are of the order of about 10(12) cells, and therefore about 10(9) to 10(10) new leukemic cells would be generated daily. Such rates of cell division are sufficient to permit more dangerous clonal variants to emerge and to influence clinical course and outcome over time.