ERBB2 and breast cancer: The phase 2 PERTAIN trial (258 patients) showed that for patients with HER2+, HR+ metastatic or locally advanced breast cancer, dual HER2-targeted therapy offers a significant PFS benefit compared to trastuzumab alone when combined with standard endocrine therapy (an aromatase inhibitor) (18.9 months vs. 15.8 months; HR 0.65, 95% CI, 0.48–0.89, p = 0.007) [47].