Fig. 1. miRNA gene regulatory networks and unique miRNA features. (A) Control of gene expression involves transcriptional, miRNA-mediated post-transcriptional and translational regulation. The selective expression of primary miRNAs and regular messenger RNAs (mRNAs) starts at the transcription step. mRNAs are further post-transcriptionally regulated by miRNAs through base-pairing to complementary sequences within the 3' untranslated regions (UTRs) of the mRNAs, resulting in mRNA degradation and/or translational repression. (B) Features of miRNAs. Large numbers of miRNAs, which are highly conserved across species, have been identified and many of them are abundantly expressed. A single miRNA is thought to have dozens of targets and one particular mRNA can be regulated by multiple miRNAs simultaneously. ORF, open reading frame.