Fig. 1

SnoRNAs exist as two main classes, each of which can be further subclassified into families. A Box C/D snoRNAs are characterized by the presence of boxes C and D found respectively near their 5′ and 3′ termini and interacting through non-canonical base pairing forming a k-turn. Additional boxes C′ and D′ can be found in the middle of the molecule. The antisense element, or guide region, base pairing with the target and specifying the residue to be methylated (Me) is found immediately upstream from the boxes D′ and/or D. B Box H/ACA snoRNAs consist of two hairpins separated by a box H (where N represents any nucleotide) and terminated by a box ACA found 3 nucleotides before the 3′ end of the molecule. The guide regions, specifying the position in the target to be pseudouridylated (Ψ) are found in bulges in the hairpins. C As genomes evolve over time, sequence duplication of snoRNAs, through recombination and retrotransposition mechanisms, can result in multiple copies of a parental snoRNA. Depending on the genomic context of the snoRNA copy, it can be expressed at different levels, which will likely affect the pressure under which it will be to retain its parental copy’s sequence. D Table indicating the number of families of different sizes for both C/D and H/ACA families in human, based on Rfam classification