PROS1 and COVID-19: This process is dependent upon the receptor binding domains (RBDs) of the trimeric S protein [12, 13], and antibodies to the S protein RBD [14] or antibodies to S protein from convalescent COVID-19 patients [11] as well as protease inhibitors that inhibit S protein cleavage into its S1 and S2 subunits [12] were all protective in preclinical models.