Aberrant Notch1 signaling is implicated in several types of cancer. Therefore, Notch signaling pathways are important anticancer targets. Pan–Notch receptor inhibition is associated with numerous complications; thus, selective Notch receptor inhibition has been pursued. Studies have shown minimal side effects with short-term blockade of either Notch1 or its ligand Delta-like 4, but long-term side effects were not investigated. In this issue of the JCI, Liu et al. use mouse models to demonstrate the consequence of long-term Notch1 inhibition. They present evidence that chronic Notch1 inhibition leads to vascular tumors in the liver and decreased survival, which suggests that Notch1 therapies should be reevaluated.