FIGURE 1. MicroRNA production.

The precursor of an miRNA (pri-miRNA) is transcribed in the nucleus. It forms a stem–loop structure that is processed to form another precursor (pre-miRNA) before being exported to the cytoplasm. Further processing by the Dicer protein creates the mature miRNA, one strand of which is incorporated into the RNA-induced silencing complex (RISC). Base pairing between the miRNA and its target directs RISC to either destroy the mRNA or impede its translation into protein. The initial stem–loop configuration of the primary transcript provides structural clues that have been used to guide searches of genomic sequence for candidate miRNA genes.