TTF1 and lung adenocarcinoma: Regarding the expression of the lung adenocarcinoma markers thyroid transcription factor 1 (TTF1) and cytokeratin 7 (CK7), the adenocarcinoma-derived cell line HCC827 harboring an activating EGFR mutation well correlates with clinical adenocarcinoma specimens, whereas the KRAS mutated widely used cell line A549 isolated from an alveolar cell carcinoma is negative for both markers (Figure 1).