CXCR4 and B-cell chronic lymphocytic leukemia: Based on our observation that the PF can be absent in the LN of a CLL patient with aggressive disease and expanded in the PB, we propose that stimulation of CXCR4hi cells in tissue leads to downregulation of CXCR4 and LN egress, giving rise to the CXCR4lo PF phenotype in the PB.