Idealised secondary structural representation of an H/ACA family snoRNA. H/ACA snoRNAs generally form a double hairpin structure. Each hairpin carries an internal bulge, called a pseudouridylation pocket, which may possess regions of base complementarity to a target RNA (not all snoRNAs carry two functional pseudouridylation pockets). We have designated these regions ψ1–4 for descriptive purposes. Other key motifs are the H-box, located in the hinge region between the two hairpins, and the ACA box at the 3' end of the molecule.