Summary of RNAi. In the cytoplasm, dsRNAs are cleaved by Dicer to generate siRNA duplexes with 5′ monophosphates, 2-nt 3′ overhangs. The newborn siRNA duplex is loaded into the Argonaute-containing, RNA-induced silencing complex (RISC). The RISC-loading complex senses the thermodynamic asymmetry of the siRNA duplex and determines the identities of the guide (black) and passenger (blue) strands. The passenger strand is removed by the endonucleolytic activity of Argonaute protein, whereas the guide strand is retained in the mature RISC and directs cleavage of the complementary mRNA targets.