Phylogenetically, snoRNAs have been found throughout the eukaryotes, including specific examples from mammals (humans, rodents, pigs), other vertebrates (chickens, Xenopus, fish, snakes), metazoan invertebrates (Drosophila melanogaster, Caenorhabditis elegans), plants (Arabidopsis thaliana, rice, corn, potato), yeasts (budding yeast, fission yeast), and protists (trypanosomes, Euglena gracilis, Chlamydomonas reinhardtii, Dictyostelium discoideum). The majority of snoRNA research has been performed in mammalian or yeast systems - of the roughly 400 snoRNAs currently in the Genbank database [Benson et al., 1999], over half are from these two groups. Since the finding of snoRNAs in early branching protists such as Euglena gracilis [Greenwood et al., 1996] and trypanosomes [Levitan et al., 1998,Roberts et al., 1998,Dunbar et al., 1999], it is expected that snoRNAs will be found in all eukaryotes. In contrast, snoRNAs have not been found in any bacteria or archaea (prior to work carried out in this thesis).