ERBB2 and breast cancer: Immunohistochemistry revealed that HER3 was overexpressed in 60% of brain metastases in breast cancer patients [45], whereas tissue microarray analysis showed that 57.6% of brain metastases in patients with various types of solid tumor—including HER2+ and HER2− breast cancer, lung cancer, and colon cancer—were positive for phosphorylated (activated) HER3 [46].