Figure 1. Canonical pathways for miRNA maturation and loading of the RISC.

After transcription, an initial pri-miRNA transcript is subject to microprocessing by the Drosha/DGCR8 complex. The resultant intermediate precursor, pre-miRNA, is a ˜70-nt hairpin that is exported from the nucleus into the cytoplasm in a regulated manner by Exportin 5. Assembly of the Dicer complex/miRNA-RISC-loading complex is required for further processing and for AGO loading. Upon disassembly of the RISC-loading complex, AGO is the chief protein factor of an active RISC, which is programmed with a specific miRNA guide strand.