Human mucin (MUC) 5B gene expression in human airway epithelium was studied in both tissue sections and cultures of tracheobronchial epithelial (TBE) cells. In situ hybridization demonstrated that MUC5B message was expressed mainly in the mucous cells of submucosal glands of normal human airway tissues. Nevertheless, an elevated MUC5B message level could be seen in surface goblet cells from patients with airway diseases and inflammation. Regardless of the airway tissue sources, MUC5B message was regulated by all-trans-retinoic acid (RA) and culture conditions in both primary and passage-1 cultures of TBE cells. MUC5B message, to a lesser extent, was also found in the immortalized epithelial cell line HBE1, but not in BEAS-2B cells. To elucidate the molecular mechanism of MUC5B gene expression, a genomic clone was obtained and sequenced for the amino terminal and the 5'-flanking region of MUC5B gene. A luciferase reporter construct containing 4,169 base pairs of the 5'-flanking region of MUC5B gene demonstrated a cell type-specific basal promoter activity in transfection studies. Both RA and the air-liquid interface culture condition further enhanced this promoter activity. These results suggest that the 5'-flanking region of MUC5B gene contains cis-elements that are potentially involved in the regulation of MUC5B gene expression.