TTF1 and non-small cell lung carcinoma: Based on immunohistochemical features, NSCLC is classified into the following pathological subtypes: adenocarcinoma, squamous cell carcinoma, adenosquamous carcinoma, sarcomatoid carcinoma and large-cell carcinoma.[2] Thyroid transcription factor-1 (TTF-1) and p40 are the most specific indicators used to distinguish adenocarcinoma from squamous cell carcinoma in the lung.[3] Adenosquamous carcinoma can be identified based on the expression of TTF-1 and p40 by different tumor cell populations.