The normal level of ACE2 is important to protect vital organs; However, as demonstrated in the models of acute lung injury (ALI) and ARDS [53, 62], the abnormal ACE2 levels were suggested to aggravate COVID-19 via the renin-angiotensin system (RAS), including promoting pathological changes in ALI [62] and being involved in inflammatory and fibrotic responses [141]. This evidence concerns the gene REN and COVID-19.