To better reveal the rising trends of those recently emerged types of RNA, the percentage of document publications of a specific year was calculated within the given type of RNA over the time (Figure ). Although the cumulative publication numbers for circRNA, exosome RNA, lncRNA, and CRISPR, are relatively small compared with others (Figure ), their rates of increase are much faster. Functions of the naturally occurring RNA types mRNA, which was the first RNA to be characterized, is the initial transcription product of a protein-coding gene and includes both protein-coding exons and non-coding introns. The mature, translatable mRNA must be spliced to remove the introns and, in transcripts that can go through alternative splicing, one or more potential exons. The mature RNA has a 5'-7-methylguanosine cap, a 5'-untranslated region, a start codon (unique sequence of 3 bases) for the translated region of the gene, a stop codon that ends the translated region of the gene and starts the 3'-untranslated region, and a 3'polyadenosine tail. Gene expression, which is often regulated by the amount of mRNA for the gene, is controlled by the balance between synthesis and degradation of mRNA. Although mRNA is a critically important RNA, it makes up only 1-5% of the RNA in a cell. ncRNAs, in contrast to mRNA, are the final functional gene products. Although it was thought initially that ncRNAs were nonfunctional junk RNA, in the 1950s, in the same paper that introduced the phrase "Central Dogma", Francis Crick correctly hypothesized that the ncRNA might function in the translation of mRNA into protein. In 1955, George Palade identified ribosomes as a small particulate component of the cytoplasm that contains RNA, and in 1965, Robert Holley purified a tRNA from yeast and determined the structure. In the last half-century, many types of ncRNA with various functions have been identified; many are involved in regulating transcription and protein expression in the cell. rRNA, which constitutes up to 80% of the RNA in an active cell, comprises three rRNAs (the 5S, 5.8S, and 28S) complexed with many proteins to form the large subunit of the ribosome and one rRNA (the 18S) complexed with proteins to form the small subunit of the ribosome. There are also two mitochondrial rRNA genes (the 12S and 16S) which, along with many proteins, form the mitochondrial ribosome. rRNA in the ribosome, acting as a ribozyme, catalyzes peptide bond formation between two amino acids. Synthesis of the large amount of rRNA occurs in the nucleolus, a heterochromatic region found in most nuclei. tRNA, which makes up 10-15% of the RNA in the cell, translates the mRNA codon sequence for each amino acid. The many different tRNAs, which are usually 75-95 nucleotides, all fold into very similar three-dimensional structures. The 3D structure exposes three unpaired nucleotides that serve as the anti-codon to base pair with the mRNA. Specific amino acids are covalently bound to a tRNA by aminoacyl tRNA synthetases. The specificity between the anti-codon and the bound amino acid is the basis of translation. Each tRNA anticodon can bind to several different mRNA codons. This pairing is based on the wobble rules for the third nucleotide position of the anti-codon, which is often posttranscriptionally modified to allow for wobble-pairing. Common modifications at the third position include 5-methyl-2-thiouridine, 5-methyl-2'-O-methyluridine, 2'-O-methyluridine, 5-methyluridine, 5hydroxyuridine, hypoxanthine, and lysidine. snRNAs ( ̴ 150 nucleotides) are components of the small nuclear ribonucleoproteins (snRNPs) of the spliceosome. They act as catalysts that splice mRNA into its mature form and they are important in the selection of alternative splicing sequences. snoRNAs (60-300 nucleotides) are bound to four core proteins and act as guides to correctly target modifications for the maturation of rRNA. They comprise two classes of RNA. C/D snoRNAs participate in the 2-methylation of targeted nucleotides, while H/ACA snoRNAs participate in the modification of uridine to pseudouridine. They help guide the protein to the specific target, rather than catalyzing the reaction directly. As participants in rRNA maturation, snoRNAs are found in the nucleolus. 35, 37-44 Exosomal RNAs are mRNAs, miRNAs, siRNAs, and lncRNAs that are packaged and exported from the cell through the exosomal pathway. Although they are poorly understood, exosomal RNAs may serve as signaling molecules to regulate gene expression in target cells. These circulating RNAs, especially miRNAs, may serve as diagnostic and/or prognostic targets for various diseases such as cancers. Types of RNA used in medical applications, their advantages, and challenges