MicroRNAs in Gene Regulation: When the Smallest Governs It All

Encoded by the genome of most eukaryotes examined so far,microRNAs (miRNAs) are small ~21-nucleotide (nt) noncodingRNAs (ncRNAs) derived from a biosynthetic cascade involvingsequential processing steps executed by the ribonucleases(RNases) III Drosha and Dicer. Following their recentidentification, miRNAs have rapidly taken the center stage as keyregulators of gene expression. In this review, we will summarizeour current knowledge of the miRNA biosynthetic pathway and itsprotein components, as well as the processes it regulates viamiRNAs, which are known to exert a variety of biological functionsin eukaryotes. Although the relative importance of miRNAs remainsto be fully appreciated, deregulated protein expression resultingfrom either dysfunctional miRNA biogenesis or abnormal miRNA-basedgene regulation may represent a key etiologic factor in several,as yet unidentified, diseases. Hence is our need to better understandthe complexity of the basic mechanisms underlying miRNA biogenesisand function