(A) Canonical box C/D snoRNAs are characterized by the presence of a terminal stem, sequence motifs (boxes C, C′, D′ and D) and guide regions with complementarity to target sequences in rRNA. The target residue base pairing with the fifth nucleotide upstream of the box D′ or D is methylated (shown by the m in a purple hexagon). (B) Canonical H/ACA snoRNAs consist of two tight hairpins separated by a hinge region (box H) and terminated by an ACA box found 3 nucleotides before the 3′ end. The hairpins typically have a bulge which is where the region of complementarity to the rRNA target is located. The uridine residue that is pseudouridylated is represented by a red Ψ.