Importantly, infection of mice with virus producing the 1968 PB1 protein, but a truncated 34 AA PB1-F2 protein, showed significantly reduced cellular infiltrate compared to infection with virus expressing the unaltered 1968 PB1/PB1-F2 proteins, implicating the PB1-F2 protein in mediating the heightened innate cellular response. The gene discussed is F2; the disease is infection.