Upon intranasal infection of mice with mouse-adapted PR8M (A/PuertoRico/8/34 H1N1 Münster variant, [15]), wild type mice lost weight significantly after infection and 50% of infected mice died, whereas Tmprss2 knock-out mice did not exhibit body weight loss and showed no signs of disease (Figs. 1A; Fig. S1). The gene discussed is TMPRSS2; the disease is infection.