High-resolution structures of assembled archaeal box C/D sRNP have been published (20–22) with some debate as to whether the complex is a dimer or monomer. It is worth noting that the only eukaryotic box C/D snoRNP structure published so far, the U3 snoRNP, is clearly a monomer (23). From the archaeal structures, however, it is clear that L7Ae and Nop5 contact the C/D and C΄/D΄ motifs with the C-terminal domain of Nop5 specifically contacting stem II (22). The N-terminal domain of Nop5 interacts with fibrillarin and this unit, referred to as the catalytic module, is mobile and docks onto the sRNA–rRNA duplex upon substrate binding. Fibrillarin binds nucleotides 1–6 of the guide-substrate duplex and a series of non-specific contacts with L7Ae and Nop5 lead to the correct positioning of the active site of this methyltransferase (22). The flexibility/movement of the catalytic module is likely important for binding and release of the modified rRNA. Based on the sequence homology of the various components and the work performed so far, the eukaryotic box C/D snoRNPs are predicted to have a similar structure to that seen for the archaeal sRNPs (18).