miRNAs and siRNAs: differences in biogenesis and properties. siRNAs (left) derive from long endogenous dsRNA molecules that form either long hairpins or bimolecular duplexes. Processing of these dsRNA precursors can generate many different siRNAs from both strands. In contrast, processing of the shorter hairpin structures known as pre-miRNAs (right) produces a single miRNA molecule from one arm of the hairpin precursor. siRNAs recognize their target transcripts with perfect sequence complementarity (left), whereas miRNAs typically have a limited number of mismatches with their mRNA target sequences (right). Both classes of small regulatory RNA molecules cause posttranscriptional silencing of protein-coding genes.