Newly made messenger RNAs are processed and packaged into compact ribonucleoprotein complexes (mRNPs) and recognized by the essential transcription-export complex (TREX) for nuclear export. However, the mechanisms of mRNP recognition, compaction, and three-dimensional organization are poorly understood. Here we perform crosslinking MS on Sulfo-SDA crosslinked endogenous ribonucleoprotein complexes (mRNPs). These have been enriched through a pulldown of THOC1-3C-GFP.