Fig. 7. A model for miRNA biogenesis. miRNA genes are transcribed by an unidentified polymerase to generate the primary transcripts, referred to as pri-miRNAs. Illustrated in the upper left is the clustered miRNA, such as miR-23∼27∼24-2, of which the pri-miRNA is polycistronic. Illustrated in the upper right is the miRNA, such as miR-30a, of which the pri-miRNA is monocistronic. The first-step processing (STEP 1) by an unknown factor(s) results in pre-miRNAs of ∼70 nt, which are recognized and exported by an unidentified export receptor. Upon export, Dicer, and possibly other factors, participates in the second-step processing (STEP 2) to produce mature miRNAs. The final product may function in a variety of regulatory pathways, such as translational control of certain mRNAs. The question marks indicate unidentified factors.