PseudoUridine (abbreviated by Ψ) or 5-ribosyluracil, is an isomer of uridine (U), and unlike uracil in uridine, which is linked to ribose by a carbon-nitrogen bond (C1-N1), uridine and ribose of Ψ are linked by a carbon-carbon bond (C1-C5). Uracil and ribose are connected by a C1-C5 bond in Ψ, so its structure is more flexible and has an additional site for hydrogen bonding than uridine, and the overlap effect in RNA is higher than that of uridine. Ψ was first discovered as early as 1951. It is well known that Ψ is the most common modification in cellular RNA and is found in all domains of organisms. Therefore, Ψ is known as the "fifth nucleoside" in addition to A, U, C, and G.