In conclusion, STAT5A, an important transcription factor for controlling mammary cell fate, is transcriptionally activated by Notch3 to suppress the metastasis of breast cancer cells, providing novel insight into the complex regulation of EMT and tumor-infiltrating immune cells, and identifying a Notch3/STAT5A pathway as a promising candidate to affect prognosis of breast cancer. Here, NOTCH3 is linked to breast cancer.