Heterogeneous FCRL protein expression on CLL cells correlates with IGHV mutation status. (A) FCRL expression varies between CLL samples. Purified mononuclear cells from the blood of 2 patients with CLL, representing the UM-CLL (CLL08) and MT-CLL (CLL32) subtypes, were stained with discriminating mAbs (black line) to define FCRL1, FCRL2, FCRL3, and FCRL5 expression on the expanded CD19⁺CD5⁺ CLL population. Isotype-matched control staining is indicated by histograms with a gray shade. Numerical values indicate the MFI ratio (top number) and percentage of the population staining positive (bottom number). (B) FCRL surface expression correlates with IGHV gene mutation status. MFI ratio comparisons determined for FCRL1, FCRL2, FCRL3, and FCRL5 expression on CD19⁺CD5⁺ cells from 55 MT-CLL samples (◇), 52 UM-CLL samples (□), and CD19⁺ B cells from 10 healthy volunteers (NL; ○). *P < .05 compared with UM and NL; #P < .05 compared with NL. The characteristics of the patient samples are described in Table S1.