miRNAs were discovered in the early 1990s and evidence supporting their involvement in the regulatory circuits of eukaryotic cells has grown rapidly. Each miRNA is thought to regulate multiple genes, and since hundreds of miRNA genes are predicted to be present in higher eukaryotes the potential regulatory impact of miRNAs is enormous. miRNAs have been found to act as key regulators of biological processes as diverse as early development, cell proliferation and cell death, apoptosis and fat metabolism, and cell differentiation. Recent studies of miRNA expression implicate miRNAs in, for example, brain development, chronic lymphocytic leukemia, colonic adenocarcinoma, Burkitt’s Lymphoma, and viral infection suggesting possible links between miRNAs and viral disease, neurodevelopment, and cancer.

microRNAs are first expressed as part of transcripts termed primary microRNAs and then processed enzymatically to generate pre-microRNA molecules [1]. After transport into the cytoplasm [2] a second enzymatic process produces active microRNA molecules that can interfere with gene expression by inducing targeted mRNA degradation (c. f. RNAi) or by active repression of the translation machinery of the cell [3].