Analysis of key ICC marker expression profiles revealed that lung adenocarcinoma in serous effusions showed high expressions of thyroid transcription factor-1 (TTF-1) and Napsin A (positivity rates: 93.0% and 76.2%, respectively); lung squamous cell carcinoma characteristically expressed P40 and P63 (positivity rates: 60.0% and 73.7%, respectively); and small cell carcinoma strongly expressed Syn and CD56 (positivity rates: 87.0% and 81.8%, respectively). The gene discussed is NKX2-1; the disease is small cell carcinoma.