CTSL and influenza: In turn, the overexpression and knockdown in vitro and the use of CatL inhibitor in vivo in mice further confirmed the promotion of CatL to ensure coronavirus entry.321 Meanwhile, CatL has been demonstrated to not only suppress viral entry but also to interrupt the life cycle of the virus.321 Additionally, a majority of available CatL irreversible or reversible inhibitors have been successfully synthesized.322 Amantadine, an antivirus drug, is used and licensed to treat influenza.