Notably, we observed that p21 knockdown significantly upregulated the expression of NP mRNA in AH1 virus-infected A549 cells (Fig 1A). To confirm the induction of p21 in response to IAV infection, A549 cells were infected with AH1 virus, then, the expression of p21 mRNA was determined at different time points post-infection. The gene discussed is CDKN1A; the disease is infection.