PROS1 and COVID-19: Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19, encodes an extensively glycosylated spike (S) protein that protrudes from the viral surface.1, 2 The S protein is a trimeric class I fusion protein, composed of two functional subunits, S1 and S2, which are responsible for receptor binding and membrane fusion, respectively.1, 3 The S protein binds to cell surface angiotensin-converting enzyme 2 (ACE2) which initiates host-cell entry and results in membrane fusion.4