Figure 1 Mechanism of Dicer-substrate small-interfering RNA (DsiRNA) processing in mammalian systems. After cellular uptake DsiRNAs are processed into 21-base siRNA, and benefit from preassociation with the enzyme Dicer to facilitate loading into RNA-induced silencing complex (RISC). Asymmetric 27-mer duplexes are incorporated in the RNA interference processing machinery one step earlier than with classical 21-base siRNA. The asymmetric design with a single two-base 3′-overhang on the antisense strand and DNA bases on the opposing blunt end provides Dicer with a substrate that cleaves in a predictable way. During RISC assembly, one the two siRNA strands, referred to as the passenger strand, is cleaved and released, whereas the other strand (guide strand) is incorporated into the argonaute protein Ago2 component of RISC. The remaining singlestranded siRNA then guides the RISC complex to its complementary target mRNA, which is finally cleaved by the endonucleolytical activity located in the Ago2 protein. ADP, adenosine diphosphate; ATP, adenosine triphosphate.