PROS1 and infection: Cellular infection with SARS-CoV-2 is initiated by the binding of the surface unit S1 of the viral spike glycoprotein to its cellular receptor angiotensin-converting enzyme 2 (ACE2), resulting in the cleavage of the viral spike-protein by the activity of transmembrane serine protease 2 (TMPRSS2) and cathepsin B and L and in viral entry [10–13].