ACE2 and COVID-19: It is worth noting that mouse is a common model species for many human medical studies, including COVID-19; however, mice and rats are the only mammals that are different from human in ACE2 protein sequences at the two virus-binding hotspots, which is consistent with previous report that SARS-CoV-2 could not use murine ACE2 protein as a receptor to enter murine cells [2].