The box C/D or C΄/D΄ motif with an adjacent guide region has been shown to direct the modification of a single nucleotide in the rRNA five base-pairs away from the D/D΄ box (Figure 1A). This is performed by positioning of the active site of Nop1/fibrillarin, which binds the C/D or C΄/D΄ motif, at the nucleotide to be modified (Figure 1B). However, the C΄/D΄ motifs and respective guides of the naturally occurring yeast snR13, snR48, U18 and U24 snoRNAs have been shown to direct the modification of two nucleotides in the same target region of the rRNA (Figure 1C). For U18, U24 and snR13 these nucleotides are adjacent, while for snR48 the nucleotides are separated by one nucleotide. In each case, deletion of the snoRNA results in the loss of both modifications (25,26). The C΄/D΄ motifs of these snoRNAs, each of which deviates from the consensus ((24) C΄ – RUGAUGA; D΄ – CUGA), could result in the modification of multiple sites within the rRNA target region (Figure 1C). It is important to note that while motifs resembling the consensus D΄ motif could be found in snR13, U18 and U24, no such motif could be found for snR48 and the conserved region we have designated the D΄ box (see later) is six nucleotides long. However, it is also possible that other mechanisms, such as a second methytransferase that recognises the primary snoRNP-catalysed modification and methylates after this has occured, could generate the modification profiles seen with the U18, U24, snR13 and snR48 snoRNPs.