A single immunoreactive band of the appropriate molecular mass for the dopamine D1 receptor (75 kDa) was identified in freshly dissected human tracheal epithelium, human pulmonary muco-epidermoid carcinoma cells (NCI-H292 cells), human bronchial epithelial cells (16HBE14o- cells), primary cultured human bronchial epithelial cells, and primary cultured human airway smooth muscle cells (positive control) (Fig. 2). This evidence concerns the gene DRD1 and squamous cell carcinoma.