CD38 and B-cell chronic lymphocytic leukemia: The levels of CD5 and CXCR4 expression have been used to define three groups of CLL cells: the CD5high and CXCR4low cells, which are recently divided cells that express high levels of activation markers including CD38; the CD5low and CXCR4high cells, resting CLL cells which are prone to re-enter lymphoid tissues; and the CD5intermediate CXCR4intermediate cells, which are in between the two other categories and are likely cells at various activation states and/or on their way to becoming quiescent [130,131].