Boulbes et al. sequenced the kinase domains of EGFR, ERBB2, and ERBB4 in 76 primary HER2+ breast cancers and found 6 mutations in EGFR, 3 in ERBB2, and 3 in ERBB4. None of the patients with ERBB mutations responded to trastuzumab, whereas 32% of patients with ERBB-WT cancers achieved partial responses. Here, ERBB4 is linked to breast cancer.