In contrast, a marked upregulation of tetherin levels on Vero E6 cells, which express endogenous tetherin (Fig. 4B), was only observed upon IFNβ treatment but not A/PR/8/34 or A/WSN/33 infection (Fig. 4C, D), in accordance with the published finding that Vero E6 cells are defective in the synthesis of IFN [25]. This evidence concerns the gene IFNB1 and infection.