Flow cytometric analysis of thebone marrow aspirate with additional markers revealed that B cells were positive forCD20 and FMC7 (relatively dim and variable) and negative for CD23 and surfaceimmunoglobulin M and immunoglobulin D. The bone marrow pathology and immunophenotypewas consistent with a diagnosis of B-PLL. This evidence concerns the gene MS4A1 and B-cell prolymphocytic leukemia.