Interferon production by virus-infected human cells. Caco-2 cells (A) and HEK 293 cells (B) were infected with SARS-CoV or the IFN-inducing control viruses Bunyamwera delNSs (BdelNSs), Sendai virus (SeV), Newcastle disease virus (NDV), or were left uninfected (mock). At 8 h (left panels) or at 16 h (right panels) post-infection, total RNA was isolated and investigated by RT-PCR for the presence of different IFN mRNAs. The cellular γ-actin mRNA served as loading control. Note that for the reliable detection of IFN-α in Caco-2 cells (A, upper right panel) the infection time had to be extended to 24 h.