A microRNA (miRNA) is a small (~22 nts) non-coding and single-stranded RNA-molecule.

Its purpose is to regulate genes by binding complementarily to them. If it binds partially, the translation of the target into a protein is repressed. After the binding the target is active again. If it binds perfectly, the target will be degraded. Usually you find this mechanism in plant organisms.

An deregulated miRNA can be used as a biomarker for distinguishing between health-states or different diseases.

You can regulate the activity of a miRNA and its target with synthetic miRNA-like molecules.

Usually a miRNA is derived from a long precursor molecule. The enzym DICER processes the precursor and the RISC complex selects if the 5p- or 3p-segment is the dominant active miRNA.