TMPRSS4 and infection: Consistent with this, TMPRSS2 and TMPRSS4 double-knockout mice show a marked reduction in the spread of H3N2 influenza virus, whereas deletion of TMPRSS4 or TMPRSS2 alone does not or only slightly protects them from death upon infection with H3N2 influenza virus, although TMPRSS2-deficient mice are protected against H1N1 influenza virus infection84.