ERBB4 and glioblastoma: During our initial IHC investigations, we noted p-ERBB4 localized on GBM vasculature, including the enlarged hyper-proliferative endothelial cell structures that are prominent GBM features (Figure 6A), and we noted ERBB4 and heregulin-1β, but not EGFR, localization (Figure S7A–D).