小分子核糖核酸, or miRNAs, are short RNA molecules that are only 21–24 nucleotides in length. The miRNAs are produced in the nucleus as longer pre-miRNAs. These pre-miRNAs fold back on themselves and form a characteristic stem-loop structure which is cleaved in the cytoplasm into mature miRNAs by an RNAse called "Dicer". Like转录因子and RNA-binding proteins, mature miRNAs recognize a specific sequence and bind to the RNA; however, miRNAs also associate with a ribonucleoprotein complex called the RNA-induced silencing complex (RISC). RISC binds along with the miRNA to degrade the target mRNA. Together, miRNAs and the RISC complex rapidly destroy the RNA molecule and regulategene expressionpost-transcriptionally.

Due to their function in post-transcriptional gene regulation, miRNAs play an important role in development. They act like switches that are able to reduce thegene expressionof several different genes at the same time. miRNAs are also associated withcancer. Often times, cancer cells alter miRNA expression levels to allow them to proliferate uncontrollably.