MicroRNA

A microRNA (miRNA) is a tiny RNA molecule, typically 21 to 24 nucleotides long, that does not code for a protein but regulates genes that do. MicroRNA genes are transcribed into longer RNAs that fold back on themselves into hairpins. These are trimmed in the nucleus, and in the cytoplasm an enzyme called Dicer cuts out a short double-stranded piece. One strand is then loaded into a protein complex called RISC, the RNA-induced silencing complex.

Guided by its microRNA, RISC finds messenger RNAs with a partly matching sequence, usually in the 3' untranslated region that follows the coding sequence, and either blocks their translation or hastens their breakdown. Because the match need not be perfect, one microRNA can dampen the output of hundreds of genes, and the human genome encodes more than a thousand microRNAs. They help steer development, cell differentiation and immune responses.

The first microRNA, lin-4, was described in 1993 in the roundworm Caenorhabditis elegans by Victor Ambros and Gary Ruvkun, who shared a Nobel Prize for the discovery in 2024. Abnormal microRNA levels are common in cancers, where some act like oncogenes and others like tumor suppressors, and microRNAs circulating in blood are being studied as markers of disease.