Flow cytometry analysis was performed to detect the binding of SARS-CoV-2 RBD protein to hACE2 receptor in 293T cells stably expressing hACE2 (hACE2/293T).26,32 SARS-CoV and MERS-CoV RBDs, as well as 293T cells stably expressing hDPP4 receptor (hDPP4/293T), were used as controls. Briefly, cells were incubated with respective RBD of SARS-CoV-2, SARS-CoV, or MERS-CoV containing a C-terminal hFc at 20 μg/ml for 30 min at room temperature, which was followed by incubation with FITC-labeled goat anti-human IgG antibody (1:500; Thermo Fisher Scientific) for 30 min and analyzed by flow cytometry. The blockage of RBD-receptor binding was performed by incubation of soluble human ACE2 (sACE2; 5 μg/ml; R&D Systems, Minneapolis, MN) receptor with respective RBD of SARS-CoV-2, SARS-CoV, or MERS-CoV (20 μg/ml), followed by the same procedure as that described above. hIgG-Fc protein (hFc: 20 μg/ml), or soluble human DPP4 (sDPP4; 5 μg/ml; R&D Systems) receptor, was included as control.