ACE2 and viral infectious disease: S1 contains the N-terminal domain and the receptor-binding domain (RBD) that contacts with the host cell surface receptor protein, angiotensin-converting enzyme 2 (ACE2).8,9 The RBD adopts either “down” (also called “close”) or “up” (also called “open”) conformation, and the ACE2 can only bind to the RBD in the “up” conformation.10 NAbs play important roles in blocking viral infection and the clearance of viral particles.