Phenotypically, these transformed B1a cells resembled CLL cells, which characteristically coexpress CD19 and CD5 (Matutes et al., 1994) together with low surface expression of B220 and IgD (Fig. 5, E and F), and thus also phenocopied the malignant CLL cells of the classical TCL1-transgenic CLL mouse model (Fig. 5 F; Bichi et al., 2002). Here, PTPRC is linked to B-cell chronic lymphocytic leukemia.