A potential mechanism of CLL development in the Eμ-TCL1 mouse involves accumulation of autoreactive B cells in the transitional T1 population, giving rise to CD19+/B220dim and, to limit autoreactivity, CD5+ B cells, which eventually progress to lethal B-cell leukemia [33–35]. The gene discussed is CD5; the disease is B-cell chronic lymphocytic leukemia.