Our results suggest that the overexpression of SERPINE1/PAI-1 reduces HA cleavage in WSN infection in the presence of trypsin, further indicating that SERPINE1/PAI-1 may target different proteases at various cellular locations, thereby inhibiting IAV haemagglutinin cleavage and reducing the infectivity of progeny viruses. The gene discussed is SERPINE1; the disease is infection.