Stereotypic (or quasi-identical) IgM BCR are known to be part of the normal adult B-cell repertoire (enriched in IgM + CD5 + B-cells in particular [32]) and, importantly, they have also been demonstrated in ~ 30% of patients with chronic lymphocytic leukemia (CLL), one of the most common IgM + mature B cell malignancies in humans [33], [34], [35], [36]. Here, CD5 is linked to B-cell chronic lymphocytic leukemia.