MicroRNAs are a recently discovered class of small, non-protein coding regulatory ribonucleic acids (RNAs) about 22 nucleotides (nt) in length whose activity and importance appear to increase in proportion to the complexity of the organism. Over 2,000 distinct human miRNAs have already been identified, and they are likely to be involved in most biological activities through regulation at both the RNA and protein levels.

MiRNAs work by binding to other RNA molecules, called transcripts, such as messenger RNA (mRNA) which acts to transmit information between the source DNA code and the intended gene products, usually proteins.  Proteins are the molecules that execute various biological functions encoded in the genome.  They are essential to all living cells as they drive  metabolic, replicative and homeostatic functions.  Proteins also comprise most of the structure of the tissues of the organism itself.

MiRNAs can bind to the mRNA transcript to “silence” its ability to make protein, or degrade the transcript altogether.  Because of its critical roles, the levels of regulatory RNAs such as miRNAs are tightly controlled by the cell.  In a disease state, for example, the levels of certain miRNAs may be increased, which suggests a positive correlation between the action of the miRNAs and pathogenic factors.  RNA therapies intend to restore the balance of a neutral state through the use of regulatory RNAs.