Gene silencing and RNA interference. (a) Dicer cleaves hairpin-shaped precursor RNAs into microRNAs (miRNAs), which bind to and silence mRNA by inhibition of translation. (b) Synthetic double-stranded RNA can also result in RNA interference (RNAi). When the double-stranded RNA is injected into a cell, Dicer cleaves it into small interfering RNAs (siRNAs), which interact with the target mRNA; the mRNA is degraded or its translation is inhibited.