Here, we present solid evidence that Notch3 can act as a tumor suppressor in breast cancer epithelial cells, that the loss of Notch3 is an important feature of TNBC, and that Notch3 inhibits EMT by activating the Hippo/yes-associated protein (YAP) pathway mediated by Kibra in breast cancer. The gene discussed is YAP1; the disease is breast carcinoma.