To investigate if CXCR4 hyperactivation alone creates a predisposition favoring TCL1-driven B-cell proliferation and CLL development, we performed B-cell immunophenotyping of CXCR4C1013G mice, focusing on transitional B cells and the population of CD19+/B220dim/CD5+ B1 B cells. Here, CD5 is linked to B-cell chronic lymphocytic leukemia.