Because infection of the mouse-adapted IFV A/PR/8/34 strain (PR8) shows similar lung pathology to human ARDS10,11,12, we intratracheally infected wild-type (WT: C57BL/6) and Card9–/–mice with a lethal dose (104 PFU/mouse) of PR8 to determine whether CARD9-mediated innate immune responses contributed to severe influenza pneumonia. Here, CARD9 is linked to pneumonia.