MX1 and influenza: The murine Mx protein has been demonstrated to be highly effective at restricting influenza, so much so that outbred mice are not natural hosts for influenza, while other species such as chickens or humans who are natural influenza hosts contain Mx homologs that are comparatively less effective, or virus strains have found ways to evade Mx antiviral action in these hosts (Pavlovic and others 1992; Pavlovic and others 1995; Schusser and others 2011).