The purified RBD proteins were analyzed by SDS-PAGE and Western blot as previously described.27,29 Briefly, proteins were separated by 10% Tris-glycine SDS-PAGE and stained with Coomassie brilliant blue or transferred to nitrocellulose membranes. The blots were blocked with 5% fat-free milk in PBS containing 0.5% Tween-20 (PBST) for 2 h at 37 °C and further incubated with SARS-CoV RBD-specific polyclonal antibody (mouse sera, 1:3,000),30 or MERS-CoV RBD-specific antibody (mouse sera, 1:3,000),31 overnight at 4 °C. The blots were then incubated with horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG (1:5,000, Thermo Fisher Scientific, Waltham, MA) for 1 h at room temperature and then visualized with ECL Western blot substrate reagents and Amersham Hyperfilm (GE Healthcare).