TMPRSS2 and COVID-19: TMPRSS2 is found in the gastrointestinal, respiratory and urogenital epithelium, but not in other extra-mucosal compartments (i.e. the heart, liver, kidneys, brain, spleen, and lymph nodes), where virus RNA or antigen is found in severe and deadly cases of COVID-19, suggesting that entry and replication of SARS-CoV-2 in these organs might be exclusively driven by endocytosis.