In that case, separation of mononuclear cells is satisfactory for most of cases at treatment initiation when the absolute lymphocyte count is usually high, while separation of CD19+ lymphocytes is performed only when the proportion of CLL cells in the sample is low (usually when ALC ≤ 10 × 109/l, depending on the detection limit of the sequencing method and the aimed cut-off). This evidence concerns the gene CD19 and B-cell chronic lymphocytic leukemia.