MicroRNAs are short RNA molecules, typically about 22 nucleotides long, that regulate gene expression after transcription. They guide protein complexes to specific messenger RNAs (mRNAs), usually by base pairing with sequences in the 3′ untranslated region. This interaction leads to mRNA degradation or inhibition of translation, effectively reducing protein production from targeted genes.

MicroRNA genes are transcribed as longer primary transcripts, processed in the nucleus by Drosha into precursor hairpins, exported to the cytoplasm, and then cleaved by Dicer into mature microRNAs. One strand is loaded into the RNA induced silencing complex, where it serves as a guide to recognize target mRNAs through partial sequence complementarity. Because a single microRNA can regulate many mRNAs, and each mRNA can be targeted by multiple microRNAs, they form dense regulatory networks.

These molecules play essential roles in development, cell differentiation, proliferation, apoptosis, and responses to stress. Specific microRNAs control processes such as muscle formation, neural development, immune cell function, and maintenance of stem cell states. Dysregulation of microRNA expression is linked to cancer, cardiovascular disease, neurodegeneration, and metabolic disorders. Some microRNAs act as tumor suppressors by limiting oncogene expression, while others function as oncogenic microRNAs, promoting malignancy when overexpressed.

Circulating microRNAs in blood and other body fluids are emerging as noninvasive biomarkers, reflecting disease states and tissue damage. Therapeutic strategies include restoring levels of beneficial microRNAs with synthetic mimics and inhibiting harmful ones with antisense molecules. Although delivery and off target effects remain major challenges, microRNA based diagnostics and treatments are actively being developed and tested in preclinical and clinical settings.