ACE2 and COVID-19: Many efforts afforded by many laboratories with various genomic approaches, aiming at converting the non-permissiveness into permissiveness of mice to SARS-CoV-2 infection, have strived and succeeded in deriving many humanized mouse models, including transient ACE2-expressing Ad5-hACE2 mice, classic human keratin (K18) and CAG promoter-driven K18- and AC70-hACE2-transgenic (Tg) mice, respectively, and various knock-in (KI) versions of hACE2-Tg mice, that have been widely used and proven useful for studies of COVID-19 pathogenesis [10, 14].