Schematic secondary structure of the two classes of snoRNAs. A) H/ACA box snoRNAs fold into "hairpin-hinge-hairpin-tail" structures and contain two short sequences called boxes H and ACA, as indicated in the white boxes. The hairpins are interrupted by an internal loop that forms the pseudouridylation pocket. A set of conserved proteins, Nhp2, Gar1, Cbf5 and Nop10, recognize and bind to this structure. B) C/D box snoRNAs show conserved boxes termed C (UGAUGA) and D (CUGA), positioned near their 5' and 3' termini. Sometimes two additional, less conserved, boxes called C' and D' are present. C and D boxes form a characteristic secondary structure: the K-turn motif. Four evolutionary conserved proteins are associated to it: Snu13 (15.5K in human), that recognizes the internal loop , Nop58 and Nop1 (Fibrillarin in human) that cross-link to the U in box C and the U in box D, respectively . In yeast, Nop58 and Snu13 are necessary for the accumulation of all C/D box snoRNAs , Nop1 is essential for the production of a subset of box C/D snoRNAs, and Nop56p is not essential for snoRNA accumulation .