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RNA editing is a term describing a variety of novel mechanisms for the modification of nucleotide sequences of RNA transcripts in different organisms. These editing events include (a) the U-insertion and -deletion type of editing found in the mitochondrion of kinetoplastid protozoa, (b) the C-insertion editing found in the mitochondrion of Physarum, (c) the C-to-U substitution editing of the mammalian apoB mRNA, (d) a similar C-to-U substitution editing of mRNAs in higher plant mitochondria and chloroplasts and in tRNAs of marsupials and rats, (e) a diverse nucleotide substitution editing of tRNAs in Acanthomoeba mitochondria, and (f) the A-to-I type of editing found in the mammalian glutamate receptor subunits. These diverse phenomena involve several different enzymatic mechanisms. In several cases, duplex RNAs with internal or external guide sequences help determine the site specificity of editing. The A-to-I editing observed in RNAs encoding non-NMDA glutamate receptor subunits may be due to the actions of a double-stranded RNA-specific adenosine deaminase that is widespread in higher organisms. Although the function of many RNA editing events is unclear, the biological importance of RNA editing in other systems may prove as significant as the nucleotide modifications regulating the cation selectivity and electrophysiological profiles elaborated by non-NMDA glutamate receptors in the mammalian brain.