ACE2 and infection: To initiate a process of infection to host cells, SARS-CoV-2 binds with its spike (S) protein to host cells first by targeting the heparan sulfate (HS) abundantly presented on the host cell surface and subsequently the angiotensin converting enzyme 2 (ACE2), eventually entering into the cytoplasm of host cells through endocytosis or fusion to the plasma membrane [25].