In accordance with the present invention, the single gene SERPINA1 has been identified as a significant predictor of survival in ER+ and ER+/HER2+ breast cancer patients. For example, patients with ER+/FIER2+ breast cancer generally have a worse outcome compared to ER+/FIER2− and ER−/FIER2+ patients. Currently there is no known predictive marker for the treatment C outcome of ER+ and ER+/FIER2+ breast cancers, thus the ability of SERPINA1 to predict the survival of this intrinsic subtype of breast cancer patients is valuable.