SLX9 is present in pre-ribosomes from an early stage and is implicated in the processing events that remove the ITS1 spacer sequences. In eukaryotes, biogenesis of ribosomes starts in the nucleolus with transcription by RNA polymerase I of a large precursor RNA molecule, called 35S pre-rRNA in yeast, in which the 18S, 5.8S, and 25S mature rRNAs reside, while RNA polymerase III transcribes a 3'-extended pre-5S rRNA. The 35S precursor also contains external transcribed spacer elements (5' and 3'-ETS) at either end as well as internal transcribed spacers (ITS1 and ITS2) that separate the mature sequences [1].

1: Slx9p facilitates efficient ITS1 processing of pre-rRNA in Saccharomyces cerevisiae. Bax R, Raue HA, Vos JC; RNA. 2006;12:2005-2013. PMID:17018574