Immunostaining of tumor cells were positive for thyroid transcription factor‐1 (TTF‐1) and cytokeratin 7 (CK7), while negative for caudal‐related homeobox transcription factor 2 (CDX‐2) and cytokeratin 20 (CK‐20) (Fig. 3B), indicating a lung adenocarcinoma origin. This evidence concerns the gene KRT7 and lung adenocarcinoma.