Common markers of resistance for epithelial growth factor receptor tyrosine kinase inhibitor drugs such as gefitinib and erlotinib for head and neck squamous cell carcinomas (HNSCC) and non-small cell lung carcinomas (NSCLC) include genes associated with the EMT, such as Trop2. Increased protein expression of vimentin combined with the loss of E-cadherin, claudin-4, and claudin-7 were associated with gefitinib resistance in both HNSCC and NSCLC cell lines. Here, CLDN4 is linked to head and neck squamous cell carcinoma.