The adaptor protein GRB2 plays a critical role in coupling signal from growth factor receptors to MAPK/ERK signaling pathway. To address the question of hPIV3-C interference with this pathway, we used a trans-reporter gene assay that measures Elk1 activation by ERK1/2. In this system, Elk1 transcription factor is fused to the DNA binding domain of Gal4 (Gal4-DB) and binds the promoter sequence of a luciferase reporter gene. Upon stimulation with a growth factor like EGF, Elk1 is activated as assessed by a significant increase in luciferase expression. Surprisingly, we observed a 6-fold enhancement in this cellular response to EGF when 36FLAG-tagged hPIV3-C was expressed in HEK-293T cells (Figure 2A ). Same results were obtained when using hPIV3-C without a tag (14-fold enhancement) or tagged with the red fluorescent protein Cherry (7-fold enhancement). In contrast to hPIV3-C, neither MV-C nor Nipah-C enhanced Elk1 activity upon EGF stimulation (Figure 2A ) whereas expression levels of hPIV3-C, MV-C and Nipah-C were similar in this system ( Figure 2F , left panel). Elk1 activity was also enhanced by hPIV3-C expression in Vero and Hela cells as well as BEAS-2B and A549, two epithelial cell lines that originate from the respiratory tract, which is the tissue targeted by hPIV3 in vivo ( Table 1 ). The effect of hPIV3-C in these different cell lines was highly significant (see p-values in Table 1 ) although relatively modest when compared to HEK-293T cells. This is probably because our reporter system requires the co-transfection of four plasmids and Vero, Hela, BEAS-2B and A549 cells are more difficult to transfect than HEK-293T. In parallel experiments, cellular response to IFN-a/b was monitored using a cis-reporter gene, of which expression is controlled by five ISREs. As previously reported [18] , we found that hPIV3-C efficiently blocked IFN-a/b signaling ( Figure 2B ) as opposed to what we observed for the EGF pathway. Again, MV-C or Nipah-C was unable to do so. Altogether, these results show that hPIV3-C enhances the cellular response to EGF in addition to its ability to block IFN-a/b signaling.