PPEF1 and infection: To assess innate immune activation in real time during infection, we generated a cell line to visualize IFIT1 transcription with single mRNA sensitivity by integrating an array of 24 PP7 binding sites (PBS) into the endogenous IFIT1 gene and expressing an mCherry-tagged PP7 coat protein (PCP), which binds with high affinity to the PBS (Methods)51.