For example, in the Up-state, the RBD of both SARS-CoV and SARS-CoV-2 is more exposed and able to bind to its ACE2 (angiotensin converting enzyme 2) receptor on the surface of human epithelial cells (type I and II pneumocytes; also, alveolar macrophage and nasal mucosal cells), whereas in the “Down-state,” the RBD is believed to be more hidden and significantly reduced to ACE2 binding and to cellular infection (12, –, 14). This evidence concerns the gene ACE2 and infection.