ACE2 and infection: Given this finding, it is possible, even likely, that there are marked genetic differences in host susceptibility that are a function of numbers or ratios of ACE2-positive AT2 cells and epithelial cells in lung (11) or the level expression by these cells of ACE2. However, we emphasize that causal linkage between genetic variation in ACE2 and to susceptibility or progress of infection is still poorly defined in humans, or even in mouse models.