In yeast, over 200 protein and RNA cofactors are required for ribosome assembly, and these are generally conserved in eukaryotes. These factors orchestrate modification and cleavage of the initial 35S precursor rRNA transcript into the mature 18S, 5.8S, and 25S rRNAs, folding of the rRNA, and binding of ribosomal proteins and 5S RNA. Nob1 is involved in pre-rRNA processing. In a late cytoplasmic processing step, Nob1 cleaves a 20S rRNA intermediate at cleavage site D to produce the mature 18S rRNA (Lamanna and Karbstein, 2009 [PubMed 19706509]).[supplied by OMIM, Nov 2010]