Porcine Mx1 blocks the influenza A virus life cycle before primary transcription. Cell monolayers were infected with the H1N1 influenza A virus strain for 3 h. Production of transcripts encoding H1N1 influenza A virus NP was quantitated by reverse transcription and real-time PCR in induced and non-induced V50 cells exposed or not, as indicated, to the translation inhibitor cycloheximide (CHX, 100 μg/mL). 18S rRNA was used as an internal control to normalize NP transcript data, and results were expressed percentages of the amount of NP transcripts retrieved from non-induced and nonexposed V50 cells, i.e. the only cells in which the virus life cycle proceeds unhindered. In the absence of CHX, poMx1 expression caused an approximately 65% reduction of NP-encoding mRNA production, and an approximately 40% reduction was still observable after exposure to CHX. Plotted values are means ± SD of triplicate real-time PCR assays, and three independent experiments were performed, with similar results. Fisher’s exact test was used to compare the means and the reported significance levels are p < 0.05 (*) and p < 0.01 (**). AU: arbitrary units. See text for detailed interpretation.