Regulatory protein of the MTREX-exosome complex involved in the synthesis of the 60S ribosomal subunit (PubMed:26456651). Participates in an early cleavage of the pre-rRNA processing pathway in cooperation with NVL (PubMed:29107693). Required for blastocyst formation, is necessary for RNA transcription, processing and/or stability during preimplantation development (By similarity). Isoform 1 interacts (through WDR repeats) with NVL; the interaction is independent of RNA or pre-60S ribosome particles. Isoform 2 does not interact with NVL (PubMed:28416111). Interacts with MTREX; the interaction dissociation in a late stage of rRNA synthesis is required for appropriate maturation of pre-60S particles and depends on the ATPase activity of NVL (PubMed:26456651, PubMed:29107693). Nucleolar location depends on active PolI transcription of pre-rRNA.