The homeodomain containing thyroid transcription factor 1 (TTF-1) is a lung- and thyroid-enriched protein implicated in the regulation of a number of pulmonary specific genes. Within the lung TTF-1 is expressed within the epithelial cells. Although the molecular mechanisms that govern this tight cell-type-specific distribution are unclear, transient transfection studies have suggested that tissue specificity is conferred in part by regions of the proximal promoter. Further studies have shown that two functionally important regions (BS1 and BS2) are sites for activation of the TTF-1 gene by the homeodomain protein HoxB3, raising the possibility that Hox proteins might function in the regulation of TTF-1 in vivo. The different cellular distributions of the two proteins within the lung suggest, however, that proteins distinct from HoxB3 might be the mediators of expression through these sites. In the present study we have used gel-mobility-shift experiments to show that in a pulmonary adenocarcinoma cell line (NCI-H441) that expresses TTF-1, the same single protein binds to both of these sites. The binding of this protein is competed for specifically by the addition of oligonucleotides containing a range of octamer-binding sites but not by a variety of non-related binding sites. Using specific antiserum we have identified this protein as being the ubiquitously expressed POU-domain protein Oct-1. Reverse transcriptase-PCR performed with degenerated primers suggests that Oct-1 is the major POU-domain-containing protein expressed in H441 cells. These results suggest that BS1 and BS2 are functional octamer sites and might therefore be implicated in the basal rather than the tissue-restricted expression of the TTF-1 gene.