NOTCH1 and B-cell chronic lymphocytic leukemia: These findings indicate that NOTCH1 activation among B cell tumors is both more sharply restricted to and more prevalent in CLL than would be predicted from prior genotypic data, raise questions about the proposed tumor suppressive role for NOTCH1 in vascular neoplasms, and suggest that IHC testing for NICD1 can be used to select patients for clinical trials of Notch pathway inhibitors, particularly those involving patients with tumors such as triple-negative breast cancer, in which NOTCH1 activation is confined to a small subset of tumors.