Pseudouridine is an isomer of the nucleoside uridine in which the uracil is attached via a carbon-carbon instead of a nitrogen-carbon glycosidic bond. It is the most prevalent of the over one hundred different modified nucleosides found in RNA. Ψ is found in all species and in many classes of RNA. Ψ is formed by enzymes called Ψ synthases, which post-transcriptionally isomerize specific uridine residues in RNA in a process termed pseudouridylation. Currently, about ∼ 9500 pseudouridine (Ψ) modifications have been identified in mammals and yeast and deposited in RMBase database.Pseudouridine is biosynthesized from uridine via the action of Ψ synthases.