KRT18 and COVID-19: Common murine models of acute COVID-19 include wildtype mice inoculated with serial mouse passaged SARS-CoV-2, engineered mice that express the human angiotensin converting enzyme 2 (ACE2) receptor under control of the cytokeratin-18 (Krt18 (K18)) promoter for epithelial cell expression, or mice that transiently express ACE2 via adenovirus or adeno-associated virus (AAV) expression systems (reviewed in (9)).