Ribosomal RNA (rRNA) is highly modified with the two most frequent modifications being 2΄-O-methyl groups and pseudouridine (Ψ) (1). 2΄-O-methylation can stabilise single base-pairs or hydrogen bonds and can strengthen or alter RNA folds while Ψ stabilises specific RNA structures (2,3). 2΄-O-methylation is found in almost all types of RNA in the cell, including tRNAs, mRNAs, small nuclear (sn)RNAs and micro (mi)RNAs (4). The majority of the 2΄-O-methyl groups are clustered in functionally important regions of the ribosome such as the peptidyl transferase domain, the decoding centre and the intersubunit bridge (1). The rRNA modification clusters are essential for cell growth (5–10) while loss of some individual modifications increases cellular sensitivity to stress and ribosome-specific antibiotics (11). The specificity of modification is important as 2΄-O-methylation of the wrong site can affect ribosome formation and/or function (12–14). Furthermore, with the recent observations that some sites in the rRNA are not 2΄-O-methylated to a 100% (15,16) and the interest in ‘specialised’ ribosomes (17), there is renewed interest in the mechanism controlling the complexity of rRNA modification.