KRT5 and squamous cell carcinoma: Although definite keratinization was not observed, immunohistochemical staining revealed that the tumor cells were positive for p40, p63, and cytokeratin 5/6 (CK5/6) and negative for thyroid transcription factor-1 (TTF-1) and napsin A, consistent with poorly differentiated squamous cell carcinoma (Figure 3).