Detection of hACE2 protein expression in hACE2/293T, or hDPP4 protein expression in hDPP4/293T, stable cell lines was performed by flow cytometry analysis, as described above, except that the cells were sequentially incubated with hACE2- or hDPP4-specific goat antibody (0.5 μg/ml; R&D Systems) at room temperature for 20 min and FITC-labeled anti-goat IgG antibody (1:200; Abcam, Cambridge, MA) for 1 h at 4 °C.