Leukemic cells retain their immunophenotypic features after long-term culture on stroma. Double-color flow cytometric dot-plots illustrate the cell marker profile of cells from case no. 3 before culture (top panels) and after 12 months of culture (middle and bottom panels). Cells after culture were obtained from clones initiated by single leukemic cells. Antibodies were conjugated to FITC (X axes) or PE (Y axes). Most leukemic cells expressed CD19, cytoplasmic μ (cIgM), terminal deoxynucleotidyl transferase (TdT), and CD10; a proportion also expressed CD15. Expression of TdT and cytoplasmic μ was confirmed by fluorescence microscopy (not shown). Cells did not express sIg, CD13, and CD3. The immunophenotype of the other three clones was identical to that of the clones shown.