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When virus RNA is produced in the cytosol, Dicer (a RNAse III Enzyme) can cut it to short nucleotides (20-25 nt) that are called siRNA (small interfering RNA). This siRNA can be incorporated into the RNA-induced silencing complex (RISC). RISC can now knockdown mRNA that has the (complementary) sequence of the siRNA. My question: Does RISC knocks down every mRNA? Does it know down the "good" cellular mRNA-transcripts or just the "bad" virus mRNA? I mean, why does RNA interference exists at all?