• naturally-occurring small segments of RNA (approximately 18-25 nucleotides in length), which play an important role in gene regulation (i.e., "turning on" or "turning off" genes in DNA) via binding-to and impacting the translation of specific mRNAs. Micro-RNAs thereby help regulate an organism's early development, cell differentiation, apoptosis, immune system, etc. Micro-RNAs were proven by Ji- Young Lee IN 2010 to routinely move between cells as a regulatory signaling molecule. The first micro-RNA was discovered by Victor Ambros in 1993.

  Micro-RNAs are directly coded-for by an organism's genes. For example, in animals the enzyme known as Drosha subsequently recognizes the micro-RNA (i.e., within an RNA hairpin loop) and cuts it out of applicable RNA transcripts. That cut-out RNA segment is then transported within the cell (to relevant DICER enzyme) where it is cleaved into the same length (21 to 24 nucleotides long) as siRNA. It is estimated that approximately 50% of human genes are regulated by micro-RNAs.

  or

• small chemically-synthesized RNA segments, which are designed to mimic the products (i.e., siRNA segments) of Dicer enzymes, in terms of causing RNA interference.

Some malfunctions of micro-RNAs can lead to some cancers or dementia diseases.

Research indicates that micro-RNAs can also cause:

• some genes within certain invading (e.g., pathogenic) viruses to shut down (via RNA silencing); thereby preventing infection of the host.
• some genes within a plant's DNA to revert to [grandparent's] wild type version of the gene, when both parents' DNA contained a mutated version of the gene.
• inhibition of expression of some cancer-causing enzymes. For example, the micro-RNA known as miR-101 inhibits the histone methyltransferase known as EZH2 (enhancer of zeste homolog 2). When certain tumors (e.g., prostate cancer) lose miR-101 via genomic deletion, then EZH2 can be over-expressed (resulting in an epigenetic change that leads to cancer metastasis).

RIBONUCLEIC ACID (RNA),  MESSENGER RNA (mRNA),  TRANSLATION,  DEOXYRIBONUCLEIC ACID (DNA),  GENE,  GENE EXPRESSION,  REGULATORY GENES,  RNA INTERFERENCE (RNAi),  EPIGENETIC,  ORGANISM,  EMBRYOLOGY,  DIFFERENTIATION,  CELL DIFFERENTIATION,  WILD TYPE,  MUTATION,  CODING SEQUENCE,  APOPTOSIS,  DICER ENZYMES,  SHORT INTERFERING RNA (siRNA),  PATHOGEN,  VIRUS,  PARAMUTATION,  HAIRPIN LOOP,  ENZYME,  INHIBITION,  HISTONES,  METHYLATED,  HOMOLOGOUS (CHROMOSOMES OR GENES),  CANCER,  CANCER EPIGENETCS,  METASTASIS,  miRNA GENES,  SIGNALING MOLECULE