A guide RNA (gRNA) is a piece of RNA that functions as a guide for RNA- or DNA-targeting enzymes, with which it forms complexes. Very often these enzymes will delete, insert or otherwise alter the targeted RNA or DNA. They occur naturally, serving important functions, but can also be designed to be used for targeted editing, such as with CRISPR-Cas9 and CRISPR-Cas12. History RNA-editing Guide RNA was discovered in 1990 by B. Blum, N. Bakalara, and L. Simpson in the mitochondria of protists called Leishmania tarentolae. The guide RNA there is encoded in maxicircle DNA and contains sequences matching those within the edited regions of the mRNA. They enable the cleavage, insertion and deletion of bases. Guide RNA in Protists Trypanosomatid protists and other kinetoplastids have a novel post-transcriptional mitochondrial RNA modification process known as "RNA editing". They have a large segment of highly organized DNA segments in their mitochondria. This mitochondrial DNA is circular and is divided into maxicircles and minicircles. A cell contains about 20-50 maxicircles which have both coding and non coding regions. The coding region is highly conserved (16-17kb) and the non-coding region varies depending on the species. Minicircles are small but more numerous than maxicircles. Minicircles constitute 95% of the mass of kinetoplastid DNA. Maxicircles can encode "cryptogenes" and some gRNAs; minicircles can encode the majority of gRNAs. As many as 1000 gRNAs can be encoded by 250 or more minicircles. Some gRNA genes show identical insertion and deletion sites even if they have different sequences, whereas other gRNA sequences are not complementary to pre-edited mRNA. Maxicircles and minicircles molecules are catenated into a giant network of DNA that is situated at the base of the flagellum in the inner compartment of the single mitochondrion. A majority of the maxicircle transcripts can not be translated into proteins due to multiple frameshifts in the sequences. These frameshifts are corrected after transcription by the insertion and deletion of uridine residues at precise sites which create an open reading frame that is translated into a mitochondrial protein homologous to mitochondrial proteins from other cells. The insertions and deletions are mediated by short guide RNA (gRNAs) which encode the editing information in the form of complementary sequences (allowing GU as well as GC base pairs). gRNA-mRNA Complex The guide RNA are mainly transcribed from the intergenic region of DNA maxicircle and these are complementary to mature mRNA. It is