Next, we classified 241 respiratory cell populations from 17 independent studies based on their expression of and literature-derived associations to ACE2 (Figure 3A). Consistent with the low levels of ACE2 in respiratory tissues by bulk RNA-seq, proteomics, and IHC (Figure 2—figure supplement 2A–D), we found that ACE2 expression is detected generally in fewer than 10% of all cell types recovered from these studies. However, as mentioned above, we believe that even low ACE2 expression levels in these respiratory cells may be relevant for COVID-19 pathogenesis.