ISG20 and infection: Our data suggest that PA-X acts to target the mRNAs of type I IFN genes induced early in infection by MAVS-dependent activation of IRF3, with no effect on the mRNAs of genes which depend on IFN signalling for their induction, such as Ifna5/6 and the ISGs Ifi44, Ifit1 and Isg20. Transcriptional control of the Il28a gene, encoding IFN-λ2, has been proposed to be similar to that of the Ifna genes as opposed to Ifnb139,40.