Overexpression of both MAST1 and MAST2 kinase-containing fusions in normal epithelial cells confers a proliferative advantage, and the presence of NOTCH-containing fusions in breast cancer cell lines leads to reduced cell-matrix adhesion, substantial morphological changes, and a unique vulnerability to NOTCH signaling inhibition, suggesting these fusion events, present in at least 5–7% of breast cancer cases overall, are most likely oncogenic in nature and contribute to tumourigenesis. Here, MAST1 is linked to breast carcinoma.