ACE2 and viral infectious disease: Considering that changing the S protein expression system, whether it is a different virus infection system or a recombinant expression system, will lead to changes in the glycan type and proportion at each glycosylation site of the S protein; it is difficult to reproduce the unique S protein glycosylation of the 16HBE (ACE2) cell origin virions and demonstrate the critical role of high-mannose N1098 site using pseudo-typed virus mutation experiment.