PROS1 and COVID-19: Structural proteins S, E, and M construct the membrane of the virus.[3] By binding the S protein to a receptor called angiotensin converting enzyme 2 (ACE2), the virus enters the targeted cell in the primary step.[4] ACE2s are primarily found on the surface of ciliated bronchial cells, lung alveolar epithelial cells, and endothelial cells.[5] Thus, the most common clinical symptoms of COVID-19 are dry cough, dyspnea, and pneumonia.