ERBB4 and esophageal squamous cell carcinoma: Since afatinib is a pan-HER inhibitor, we also examined the expression levels and genetic alterations of HER2, HER3, and HER4 in these ESCC cell lines and PDX models, but all these three molecules were hardly detected by western blotting or IHC, and no genetic alterations were found to be correlated with the afatinib sensitivity (data not shown).