SnoRNAs are short molecules, generally between 60-400 nucleotides (nt) in length (yeast snR30 is exceptional at 608 nt). They are transcribed by RNA polymerase II (except MRP and plant U3, transcribed by RNA polymerase III), and can be found in several different genomic contexts. These loci include the introns of protein-coding genes (on the mRNA coding strand), polycistronic arrays of multiple snoRNAs, or single, independent transcription units. Most mammalian snoRNAs occur in introns [Smith & Steitz, 1997], and most yeast snoRNAs occur independently or within arrays. SnoRNAs that are independently transcribed have a 5' trimethylguanosine cap structure which protects them from degradation. Special nucleolytic pathways are required to free intronic and polycistronic snoRNAs for processing to mature lengths [Petfalski et al., 1998,Chanfreau et al., 1998a].