MicroRNAs (miRNAs) constitute a group of single-stranded, noncoding RNAs that regulate the expression of structural genes on post-transcriptional level. miRNAs are transcribed as several hundred- nucleotide pri-miRNAs. The mature, active form is only 21–23 nucleotides long and it binds target mRNAs containing antisense sequences. miRNA can either catalyze cleavage of mRNA that are perfectly base- paired to its sequence, or inhibit the translation of mRNAs that form an imperfect complex with the miRNA. Hundreds of miRNAs are described to date, which populate the genomes of plants and animals.