In healthy (non-smoking) individuals, ACE2 — the cell-surface receptor SARS-CoV-2 uses to enter host cells — is expressed throughout the airway epithelium. Its expression in this baseline group has been characterized along three lines: (1) expression across the tracheobronchial tree (trachea, large airway epithelium, and small airway epithelium); (2) expression in primary basal cells as they differentiate; and (3) expression by individual cell type within the small airway epithelium. Together these establish the normal airway expression pattern used as a baseline for comparison with smokers and other conditions.