Each datapoint represents a single cell. All major cell types of the small airway epithelium (SAE), including basal cells, intermediate cells, club cells, mucus cells, and ciliated cells, express ACE2. Because of technology limitations, only 1-2% of all of the tested cells were found to express ACE2. It is thought that cells expressing lower amounts of the enzyme went uncounted, and these reported data are an underrepresentation of the number of cells that express ACE2.