Figure 1.

Schematic representation of the known miRNA biogenesis pathways in mammalian cells. (A) In the canonical miRNA processing pathway, Drosha liberates the pre-miRNA by direct cleavage of the pri-miRNA. (B) In the miRtron pathway, the pre-miRNA is generated by pre-mRNA splicing. (C) The miRNA expression pathway used by MHV68 requires processing of a tRNA:pre-miRNA fusion intermediate by tRNAseZ to liberate the pre-miRNA. (D) In this final pathway, short structured RNAs, generated by Pol III transcription, are directly exported to the cytoplasm, where they are cleaved by Dicer. As shown, export is mediated by Exp5, but this may not always be the case.