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microRNAs (miRNA or μRNA) are a family of single-stranded non-coding RNA molecules of 21-23 nucleotides in length. The discovery of microRNAs added a new layer of gene expression at post-transcriptional level. The primary transcript of microRNA is a pri-miRNA, which is processed into a short stem-loop structure named a pre-miRNA. The final functional form of a microRNA is then loaded into RISC (RNA-induced silencing comples). microRNAs regulate their target, mRNAs, by perfect (mostly in plants) of imperfect (mostly in mammalian cells) complementary binding. One microRNA can regulate multiple mRNAs by repressing translation or destablizing mRNAs.