ACE2 and COVID-19: Given the expression of ACE2 in a variety of tissues, human cell-based platforms are helpful to verify whether this receptor and other host entry and restriction factors are present in SARS-CoV-2 target cells, to evaluate viral tropism, to study the cell types permissive to SARS CoV-2 infection, and to model COVID-19 pathogenesis across multiple organ systems (Figure 3).