To investigate if spleen enlargement is associated with the accumulation of cells with distinct phenotypes, we compared CXCR4 and CD5 profiles of spleen and PB CLL cells and observed an expansion of the CXCR4hiCD5hi fraction in the spleen, significantly enriched with Ki67+ cells (Supporting Information S1: Figure 3c–e). This evidence concerns the gene MKI67 and B-cell chronic lymphocytic leukemia.