CXCR4 and B-cell chronic lymphocytic leukemia: In light of our initial findings, we hypothesized that this proliferative fraction of circulating CLL cells should therefore have increased metabolic activity when compared with the CXCR4highCD5low and CXCR4intCD5int fractions, as previously defined.15 Consistent with previous reports we were able to identify three fractions based on CD5 and CXCR4 expression (Suppl.