First, Notch2 was found to be indispensable for CLL development in Cd5+ (B-1a) B-cells [12] and, second, enforced expression of truncated, constitutive active Notch2 forms led to the selective development and expansion of Cd5+ (B-1a) B-cells [13]. Here, NOTCH2 is linked to B-cell chronic lymphocytic leukemia.