Immunohistologically, the tumor cells were stained positive for cytokeratin 7 (CK7) in the cytoplasm and for thyroid transcription factor 1 (TTF-1) in the nucleus, but negative for cytokeratin 20 (CK20), suggesting that the tumor cells originated from either thyroid cancer or lung adenocarcinoma. This evidence concerns the gene KRT7 and lung adenocarcinoma.