Thyroid transcription factor-1 (TTF-1) is a protein seen on the surface of thyroid cells, but also on about 70-75% lung adenocarcinomas and only a small minority (~10%) of squamous cell NSCLC tumors. In fact, the presence of TTF-1 on a NSCLC tumor provides a good hint for the pathologist that this is an adenocarcinoma. It's an immunohistochemical (IHC) test that is done on the vast majority of lung cancers, and there's some new information that suggests it may also be useful for predicting which patients are especially unlikely to have an EGFR mutation or ALK rearrangement.