SummaryPseudouridine (5-ribosyluracil ) is a ubiquitous yet enigmatic constituent of structural RNAs (transfer, ribosomal, small nuclear, and small nucleolar). Although pseudouridine (W ) was the rst modi ed nucleoside to be discovered in RNA, and is the most abundant, its biosynthesis and biological roles have remained poorly understood since its identi cation as a " fth nucleoside" in RNA. Recently, a combination of biochemical, biophysical, and genetic approaches has helped to illuminate the structural consequences of W in polyribonucleotides, the biochemical mechanism of U! W