Figure 1.   (a) Secondary structure consensus of C/D snoRNAs. The classical drawing of helices with canonical as well as other isosteric base pairs that form the K‐turn follows [ 68]. The C′ and D′ boxes that are not always conserved (by contrast to the C and D boxes) are shown in lower‐case letters and dashed base‐pair symbols (forming preferentially a K‐loop instead of a K‐turn);   (b) 2′‐O‐ribose methylation reaction catalyzed by C/D snoRNPs;   (c) Secondary structure consensus of H/ACA snoRNAs;   (d) Pseudouridylation reaction catalyzed by H/ACA snoRNPs: a uridine (U) base is isomerized into a pseudouridine (ψ).