MicroRNAs are tiny molecules of nucleic acid that control gene expression, acting like a dimmer switch to tone down gene output at key positions in the network of information that governs a cell's function. MicroRNAs are important for the day-to-day inner working of cells and especially important during development. They also become profoundly defective in diseases such as cancer. Unlike most other human or animal genes, microRNAs are often encoded in genomes and expressed as beads-on-a-string groupings, known as polycistrons. The purpose for this organisation has, until now, been a mystery.
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