MicroRNAs are short RNA molecules, typically about 22 nucleotides long, that do not code for proteins but instead regulate gene expression. They are produced through a multi step process beginning with transcription of primary microRNA transcripts, processing by Drosha in the nucleus, export to the cytoplasm, and further cleavage by Dicer to generate mature microRNAs. These are then loaded into the RNA induced silencing complex, where they guide the complex to target messenger RNAs by base pairing, usually at the 3′ untranslated region, to repress translation or promote degradation.

A single microRNA can regulate many different target genes, and individual genes can be regulated by multiple microRNAs. This gives rise to dense regulatory networks that fine tune cellular processes such as development, differentiation, proliferation, apoptosis and responses to stress. MicroRNAs help shape cell identity, contribute to patterning in embryos, and modulate signaling pathways by dampening noise and sharpening thresholds.

Altered microRNA expression is associated with numerous diseases. In cancer, some microRNAs function like oncogenes by repressing tumor suppressor genes, while others act like tumor suppressors by restraining oncogenes. Distinct expression signatures can classify tumor types and correlate with prognosis. In cardiovascular and neurological disorders, microRNAs participate in pathological remodeling, neurodegeneration and synaptic plasticity. Circulating microRNAs in blood and other fluids are being investigated as non invasive biomarkers.

Therapeutic strategies aim either to inhibit overactive microRNAs using antisense molecules or to replace lost microRNAs with synthetic mimics. Challenges include ensuring specificity, efficient delivery and minimizing off target effects, but early studies indicate that targeting microRNA pathways could complement existing treatments across a range of human diseases.