Updated FTP location. HEK293T cells were cultured for 3hr in okadaic acid (200 nM; LC Laboratories) and subsequently collected and lysed using Hypotonic Gentle Lysis Buffer with Halt Protease and Phosphatase Inhibitor Cocktail (Thermo Fisher Scientific). Cellular p-UPF1 was immunoprecipitated using the rabbit polyclonal anti-p-UFP1 S1116 antibody (Millipore) in the presence of 1 U/ml of RNase I (Thermo Fisher Scientific Ambion). Immunoprecipitated material was electrophoresed in 12% SDS-polyacrylamide, and silver-stained bands using SilverQuest Staining Kit (Thermo Fisher Scientific Invitrogen) were excised and subjected to in-gel trypsin digestion. Chromatographed peptides were identified at the Massachusetts Institute of Technology Whitehead Institute Proteomics Core Facility (http://massspec.wi.mit.edu/) using an Orbitrap EliteTM Hybrid Ion Trap-Orbitap Mass Spectrometer with Dionex Ultimate 3000 Rapid Separation Liquid Chromatography (Thermo Fisher Scientific) and Scaffold Software (version Scaffold_4.2.1, Proteome Software Inc.).