Detection limit by direct sequencing. Gel electrophoresis of PCR products from serial twofold dilutions of a CLL sample into a normal sample (see text for more details) is shown. The fold dilution is indicated on the left. The calculated theoretical concentration of CLL cells in percent among WBC and total B cells (B) is shown on the right. The ability to identify the expected VH3-48 variable region with 99.7% identity to the germline sequence is indicated in the far right column (Y or N). The gel lanes are labeled according to the leader primer used in the PCR reaction: 1, VH1; 3, VH3; 4, VH4; C, VH2, VH3-21, VH5, and VH6 multiplex. DNA fragment length is indicated in bp.