Fig. 7. A model for the evolution of the EJC upon mRNA export. Pre-mRNA splicing deposits the EJC upstream of the splice junction. At this stage, the complex contains at least five proteins, SRm160, DEK, RNPS1, REF and Y14. After splicing, the TAP/p15 heterodimer and Upf3 join the complex in the nucleus. DEK probably dissociates (dashed arrow) before mRNA export to the cytoplasm, whereas SRm160 and the nucleocytoplasmic shuttling proteins RNPS1, REF, TAP/p15 and Upf3 are all likely to leave after mRNA export. Y14 remains stably associated with spliced mRNA in both compartments, while Upf2 only joins the complex in the cytoplasm. Question marks symbolize potential unidentified components.