MicroRNA (miRNA) biogenesis. The canonical pathway includes cleavage of pri-miRNAs in the nucleus by Drosha, whereas pre-miRNAs are processed by Dicer in the cytoplasm. Some of the miRNAs located within introns of protein-coding genes bypass Drosha cleavage. These so-called mirtrons are processed from their primary transcripts within an alternative (mirtronic) pathway by splicing and debranching. Finally, from the resulting miRNA duplex, the strand with the higher affinity is assembled into the RNA-induced silencing complex. Complementary base pairing with the target mRNA leads either to degradation of the mRNA or to translational repression, depending on the complement of the sequences. This figure has been modified according to . Ago, Agonaute proteins.