Our previous study31 showed that these TMPRSS2-KO mice are highly tolerant to lethal dose infection with influenza virus strains that use TMPRSS2 for hemagglutinin cleavage but are as susceptible as WT mice to the infection with H5N1 subtype influenza virus that uses furin, but not TMPRSS2, for hemagglutinin cleavage. This evidence concerns the gene FURIN and infection.