Thyroid Transcription Factor 1 antibody identifies the 40 kD TTF-1 band in nuclear extracts or whole cell lysates of TTF-1-positive cell lines. It is useful in differentiating primary adenocarcinoma of the lung from metastatic carcinomas from the breast. Futhermore it can also be used to differentiate Small cell lung carcinoma from lymphoid infiltrates. Loss of TTF-1 expression in non-small cell lung carcinoma has been associated with aggressive behavior of such neoplasms.