ERBB3 and head and neck squamous cell carcinoma: We confirmed that EGFR protein is only being measured in the tumor, as immunohistochemistry using an EGFR antibody cross-reactive to mouse and human in both untreated and ErbB3-antibody-treated tumors showed that only the HNSCC tumor cells and not the stroma express EGFR (Supplementary Fig. 3).