During the synthesis of eukaryotic ribosomes, four mature ribosomal RNAs (the 5S, 5.8S, 18S, and 25S) must be synthesized.  Three of these rRNAs (5.8S, 18S, and 25S) come from a single pre-rRNA known as the 35S.  Although many of the intermediates in this rRNA processing pathway have been identified in the last thirty years, there are still a number of proteins involved in this process whose specific function is unknown.