Figure 1.

The mechanism of RNAi and its off-target effect in mammalian cells. An RNA strand with A or U at position 1, four to seven A/Us in positions 1–7 and G/C at position 19 (measured from the guide strand 5′ end) are easily unwound from the 5′ end and retained in the RISC. The passenger strand is dissociated from the Ago1 ∼ 4-containing RISC following unwinding, but cleaved in the Ago2-containing RISC. The guide strand recognizes target and off-target transcripts with complementary sequences to seed region positions 2–8. The target transcript, which has complete complementarity in positions 9-21, in addition to positions 2–8, is knocked down by RNAi. Conversely, off-target transcripts are downregulated according to the thermodynamic stability in the duplex formed between the siRNA seed region and target mRNA.