MicroRNAs (miRNAs) are an abundant group of endogenous small (~22) noncoding RNAs, found both in plants and animals. The total number of sequences encoding miRNA in genomes of these organisms is estimated to be ~1% of all genes. miRNAs derive from longer precursor transcripts which adopt a fold- back structure. Mature miRNAs are incorporated into ribonucleoprotein complexes miRNPs and take part in negative control of genes' expression by cleavage of mRNAs or inhibition of the translation. miRNAs influence a great range of biological processes, such as cell proliferation, stem cell maitenance, timing of cell fate in developmental process, organ morphogenesis, programmed cell death and fat metabolism.