SnoRNAs can be isolated biochemically based on their association with one or more nucleolar proteins. Fibrillarin (NOP1 in yeast) is the best studied snoRNA-associated protein, and is one of the core components of snoRNPs that associate with C/D box snoRNAs. Fibrillarin is highly conserved throughout eukaryotes, and is required for pre-rRNA processing, pre-rRNA methylation, and ribosome assembly [Tollervey et al., 1993]. Other C/D box snoRNA-associated proteins include p68 in Xenopus [Caffarelli et al., 1998], Nop56p and Nop58p in yeast [Gautier et al., 1997], and a 65 kDa U14-associated protein in mouse [Watkins et al., 1998b]. The other major family of snoRNAs, the H/ACA box snoRNAs, were originally recognized as a group based their association with the yeast protein GAR1p. Other proteins associated with H/ACA box snoRNAs include Cbf5p (a pseudouridine synthase), Nhp2p, and Nop10p [Watkins et al., 1998a,Henras et al., 1998]. Many other nucleolar proteins are known to be essential for ribosomal RNA processing, yet direct associations with snoRNAs or snoRNA-containing snoRNPs have not been demonstrated.