Micro RNA (miRNA) are small pieces of single-stranded (ss) RNA, usually about 21 - 25 nucleotides long, which "interfere" with the translation of proteins by binding to messenger RNA (mRNA). miRNA come from longer, endogenous, RNA, called pri-miRNA, that is produced within the cell. The pri-miRNA is cleaved by RNase III, into hairpins of about 70 base pairs in length. Like siRNA, the hairpins have 3' overhangs of 2 nucleotides at each end that faciliate their incorporation into the multi-subunit protein complex called RNAi induced silencing complex (RISC), once the miRNA has been excised from the pre-miRNA by the enzyme called Dicer.
Some miRNA are known to be either very exactly complimentary to their target mRNA sequences, and capable of inducing mRNA degradation, while others are non-specific and can bind many different types of mRNA (usually at the 3' untranslated region). The non-specific miRNA are more likely to just repress translation, than induce mRNA degradation.
Small interfering RNA, like miRNA, are being studied for the role they play in different diseases. Endogenous miRNA play an important role in controlling gene expression and epigenetics, thereby influencing the growth and development of organisms.
Tsai, C.S. Biomacromolecules: Introduction to structure, function and informatics. Wiley-Liss, 2007.