Comparison of FCRL1-5 surface expression on polyclonal B cells and malignant CLL cells. Ficoll-purified mononuclear cells isolated from the blood of a patient with CLL (CLL37; Table S1) were stained with discriminating mAbs to detect polyclonal CD19⁺CD5⁻ B cells and the CD19⁺CD5⁺ leukemic CLL population in order to assess FCRL1-5, CD38, ZAP-70, and CD27 expression by multiparametric flow cytometry. Histograms reflect the staining of specific mAbs (black line) versus isotype-matched controls (gray shade) within the respective gated populations. The numbers indicated within histograms specify the MFI ratio (top values) and the percent of the population staining positive (bottom values). The MFI ratio is derived from the MFI of the test mAb divided by that of the isotype-matched control. Note that the underlined numbers in the ZAP-70 panel indicate the T/B-CLL MFI ratio.