Structure of influenza A polymerase bound to the viral RNA promoter and the action mechanism of baloxavir marboxil (BXM). (1) Influenza virus catch the host mRNA that has cap structures at the binding domain comprising subunits PA, PB1 and PB2 subunits, (2) The PA endonuclease cut and snatch the cap structures from the host mRNA (so called ‘cap snatching’ process), and (3) viral mRNA had the cap structures finally to continue the viral replications in the host cell. BXM specifically inhibit the PA endonuclease and then stop the viral replication without side effects to the host cell.