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.