Intriguingly, mRNA expression of mouse Ace2 was also downregulated (Supplementary Fig. 8e, f), suggesting that transcriptional/post-transcriptional mechanism may be involved in the ACE2 downregulation in COVID-19, such as NF-kB-induced microRNA-mediated downregulation of ACE2 mRNA41. Here, ACE2 is linked to COVID-19.