Intriguingly, mucosal ACE2 levels may also indicate a non-monotonic relationship with the severity of SARS-CoV-2 infection, due to the fact that ACE2 is not only the receptor of SARS-CoV-2, but ACE2 also suppresses the exuberant immune response that can trigger acute respiratory distress syndrome (ARDs), a major reason for Covid-19 fatalities. Here, ACE2 is linked to acute respiratory distress syndrome.