CXCR4 and B-cell chronic lymphocytic leukemia: Fluctuations in CXCR4 expression on proliferating CLL cells may mirror the changes in CXCR4 levels on GC B cells where CXCR4 expression discriminates between proliferating and nonproliferating subsets in the dark (DZ) and light zones (LZ), respectively.41, 42