ACE2 and pneumonia: This pneumonia-causing disorder spreads primarily from respiratory droplets of infected individuals in enclosed spaces to mucosal epithelial cells in the upper airway and oral cavity where it gains entry via its homotrimeric spike protein to host-cell expressed angiotensin-converting enzyme-2 (ACE2) receptor binding sites in a protease-dependent manner (Liu et al., 2020; Oberfeld et al., 2020).