NOTCH1 and neoplasm: Elegant work in the mouse epithelium demonstrates how ablation of Notch1 in the adult epithelium drives clonal expansion of Notch1-deficient basal cells during homeostasis but at the same time, impairs tumor formation (Abby et al., 2023), revealing how inactivating Notch1 mutations in human epithelia could couple progenitor cell expansion with tumor suppression.