BCR and B-cell chronic lymphocytic leukemia: However, tumor cells from both CLL groups typically display a mixed immature (CD5+ CD23+) and “antigen-experienced” (CD27+) B-cell phenotype (10), suggesting they might represent the leukemic counterpart of B-lymphocytes that might have undergone BCR stimulation in the GC (M-CLL) vs. peripheral tissues, following selection of B-cell precursors in BM (U-CLL).