ERBB2 and breast cancer: Brain metastasis is a common and feared complication of breast cancer, affecting approximately 30% to 35% of patients with advanced ERBB2 (formerly HER2 or HER2/neu)–positive (ERBB2+) disease.1 In the past decade, improving therapies have led to higher rates of durable response and significant survival benefit in ERBB2+ metastatic breast cancer (MBC).