ERBB2 and breast cancer: These results further establish that CHIP is a key regulator of oncogenic traits and therapeutic sensitivity of tumor cells, and the results described here could form the basis of future efforts to target CHIP-low ErbB2-overexpressing breast cancers with a combination of ER stress inducers such as Bortezomib together with Trastuzumab to improve the response of patients with ErbB2-overexpressing breast cancer to ErbB2-targeted therapy.