WM B cells coexpress clonotypic IgM and IgD transcripts. Representative analysis of clonal isotypes in multiple individual cells from the same patient is shown. cDNA from individual CD20⁺ cells of patient WM1-09 was split into separate aliquots, and heminested PCR was run in parallel experiments using the combination of primer sets described in Table 2. Percentages of cells expressing single or double clonal isotypes of each patient were calculated and summarized in Table 4.