Antibodies recognizing the Flag tag (Sigma,F1804), SETX (Novus, NBP1-94712), RNA polymerase I (RPA194 (C-1) (Santa Cruz, sc-48385), fibrillarin (G-8) (Santa Cruz, sc-374022), nucleophosmin (NPM1, Forties Life Sciences A302-403A-T), C1QBP/P32 (Proteintech, 24474-1-AP), CHCHD2 (Proteintech, 66302-1-lg), GAPDH (Sigma-Aldrich, G9545), U2AF65 (Sigma-Aldrich, U4758), H3 (Abcam, ab8898), phospho-histone H2A.X(Ser139) (Cell signaling, 2577), and MitoTracker Red CMXRos (Invitrogen, M7512) were obtained from the indicated vendors. Human RIEP rabbit polyclonal antibodies were generated at Bethyl Laboratories/Fortis Life Sciences. RIEP-1 and RIEP-216 antibodies recognize peptides derived from the RIEP N terminus or C terminus, respectively. IGPAL-CA-630 (Sigma, I8896), cOmplete™, Mini, EDTA-free Protease Inhibitor Cocktail (Roche, 04693159001), and Pierce™ Protein A/G Magnetic Beads (Thermo Fisher scientific, 88802) were purchased from indicated vendors.