MX1 and infection: While several important host factors have been identified, three that are well characterized are the tripartite motif-containing protein 25 (TRIM25) and the retinoic acid-inducible gene I (RIG-I), which can bind the incoming viral ribonuleoproteins and reduce infection, and the MxA protein (in humans) which is a nuclear interferon-stimulated gene (ISG) that binds to the viral ribonuleoproteins to block infection.