ACE2 and COVID-19: On one hand, the case fatality rate of COVID-19 is much lower than that of SARS (Li et al., 2020); on the other hand, X-ray crystal diffraction has implied that the combination between S protein of SARS-CoV-2 and ACE2 is a little stronger than that of SARS-CoV (Lan et al., 2020; Shang et al., 2020).