In the cytoplasm, additional regulatory mechanisms control gene expression. The stability of mRNAs in the cytoplasm is highly variable. Some are degraded rapidly, allowing for only a short period of translation, while others are quite stable. In many cases, the life span of an mRNA is controlled by RNA interference. In the process of RNA interference, specific mRNAs are targeted by tiny, single-stranded RNA molecules called microRNAs (miRNAs). After binding to proteins called a RISC protein complex, these miRNAs bind to complementary sequences in the mRNA. Once part of an mRNA becomes double stranded in this way, specific proteins degrade the mRNA or prevent it from being translated into a polypeptide. About 20-23% of all animal and plant genes are regulated by miRNAs.