TMPRSS2 and infection: The expression levels of Tmprss2 were generally low in both mouse strains before infection and at days 1 to 6 p.i., with slight but not significantly higher levels in lungs from DBA/2J mice until day 3 p.i. In both mouse strains, the levels of the Tmprss4 gene were significantly higher than Tmprss2 (P < 0.05).