ACE2 and infection: It has been shown that SARS-CoV-2 infects human cells via specific binding of S-protein to angiotensin-converting enzyme 2 (ACE2) (Figure 2 Infection and replication process of SARS-CoV-2) (Xu et al., 2020) and the binding affinity between these two proteins is 10–20 times greater than that of SARS-CoV and ACE2 (Yan et al., 2020; Wrapp et al., 2020).