Recent developments in understanding the mechanism and functions of microRNAs Suhail Muzaffar, Karishma Bisht186_pdf.pdf001-007MicroRNA, MicroRNA biogenesis, Functions of microRNAs.141168Based on their mechanism of action and biological function, several classes of small RNAs have come into the limelight in the last two decades. These small RNA molecules generally belong to three main categories: short interfering RNAs (siRNAs), microRNAs (miRNAs), and piwi-interacting RNAs (piRNAs). miRNAs and siRNAs are distinguished primarily because miRNAs are endogenous in nature and are expressed by an organism's own genome, whereas siRNAs are exogenous in origin and derived mainly from the viruses and transposons. The first miRNA, lin-4, was discovered in 1993 as an endogenous regulator of genes that control developmental timing in Caenorhabditis elegans. miRNAs are coded by both plant and animal genomes and their transcription is typically performed by RNA polymerase II. 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