The neoplastic cells were negative for thyroid transcription factor 1 (TTF-1) and cytokeratin (CK) 7, whereas they were positive for carcinoembryonic antigen (CEA) and CK20 and the protein encoded by the caudal type homeobox 2 (CDX2), therefore suggesting a colonic source contrary to the initial suspicion of lung adenocarcinoma. Here, CEACAM5 is linked to lung adenocarcinoma.