In addition to TMPRSS2, it has been reported that other TTSPs, such as TMPRSS11D (also known as HAT), TMPRSS11E (known as DESC1), and TMPRSS13 (known as MSPL), also mediate the proteolytic activation of influenza viruses and coronaviruses, whereas their roles in infection in vivo remain to be elucidated (3, 11, 12). Here, TMPRSS2 is linked to infection.