Therapeutic response of leukemic clones in patient B20. The relative levels of IgH rearrangements marking either the major diagnosis (VH2, ▴) or relapse (VH3, ●) clones were analyzed by RQ-PCR in diagnostic, follow-up, and relapse bone marrow samples. The presence of either marker could be detected at levels above 10⁻⁴. The patient suffered a CNS relapse (relapse 1) and a second bone marrow relapse. Data points below the detection limit line are arbitrarily placed and indicate points at which VH2 (▵) or VH3 (○) rearrangements were undetectable. Dotted line indicates extrapolated level of clone between 1 and 3 months.