For all virus concentrations tested, the mRNA expression for IFNB1, IFIT1 and IFIT2 were upregulated to a higher degree in the presence of Vpx than in its absence, despite comparable infection levels (Fig.  6c; 1× vs. 10×) and in some cases even though Vpx-containing virus resulted in less GFP + cells (Fig. 6c; 3× vs. 100×). Here, IFNB1 is linked to infection.