CD5 and B-cell chronic lymphocytic leukemia: The first and most used genetically engineered mouse model of CLL is the T-cell leukemia/lymphoma 1 transgenic (Eμ-TCL1) model that triggers the development of clonal CD5+ B cells in older mice, and develops an aggressive CLL variant [14, 15], lacking the recurrent driver gene mutations found in primary CLL [16].