Notably, we observed that endogenous IFITM3 expression was only transiently and moderately upregulated in A549 and 293T cells but not in HeLa after 2–6 h infection, and then shut down in a timely manner following infection with VACV at 0.1 PFU per cell (Figure 1A) in HeLa, A549, and 293T cells. Here, IFITM3 is linked to infection.