Positive staining of transfected unfixed live cells with serum from convalescent COVID-19 patients as well as with the RBD antibody CR3022, which in the context of the S protein binds to an epitope of the RBM only accessible in the up conformation [21,62] indicated that the minispike RBD acquires a conformation corresponding to that of the natural SARS-CoV-2 RBD. Here, PROS1 is linked to COVID-19.