Using flow cytometry with insect cells expressing S proteins at 2 days post-infection (p.i.), when expression levels were saturating (Fig. 1), incubation of infected cells with an excess of ACE-2–GFP resulted in a two- to threefold difference in ACE-2 binding between SARS-CoV and NL63 S1 (Fig. 3a). Here, PROS1 is linked to infection.