miRNA biogenesis. An miRNA gene is transcribed, generally by RNA polymerase II (Pol II), generating the primary miRNA (pri-miRNA). In the nucleus, the RNase III endonuclease Drosha and the double-stranded RNA-binding domain (dsRBD) protein DGCR8/Pasha cleave the pri-miRNA to produce a 2-nt 3 overhang containing the ∼70-nt precursor miRNA (pre-miRNA). Exportin-5 transports the pre-miRNA into the cytoplasm, where it is cleaved by another RNase III endonuclease, Dicer, together with the dsRBD protein TRBP/Loquacious, releasing the 2-nt 3 overhang containing a ∼21-nt miRNA:miRNA∗ duplex. The miRNA strand is loaded into an Argonaute-containing RNA-induced silencing complex (RISC), whereas the miRNA∗ strand is typically degraded.