CB-103 (218457-67-1) is an orally active inhibitor of the Notch signaling activation complex (IC50’s from 0.9 to 3.9 μM in various cell-based assays), the most downstream level of the Notch signaling pathway. It inhibited the growth of Notch-addicted human T cell acute lymphoblastic leukemia cells as well as other cell lines.? CB-103 also inhibited the growth of human breast cancer and leukemia xenografts without the intestinal toxicity associated with γ-secretase Notch inhibitors.