Finally, mRNAs compose the most heterogeneous group of RNAs, varying in length and structure. Thus, different export factors and adaptor proteins associate with each subpopulation of mRNAs [163] . mRNAs are transcribed by PolII, and, concomitantly with this process, a number of RNA-binding proteins assemble with them. These RNAbinding proteins exert different modifications in immature mRNAs such as polyadenylation, splicing and capping. In addition, export factors and adaptor proteins are also recruited to nascent pre-mRNAs, playing a further function in nuclear export. TREX (transcription-coupled export) complex is recruited to the 5 end of nascent pre-mRNAs in a splicing-dependent manner by means of the interaction of one of its components, namely ALY/REF, with the subunit of the nuclear cap-binding complex CBP80 [182] . The TREX complex also recruits the conserved RNA-helicase UAP56 that is important for mRNP biogenesis [183] . TAP-p15, (also known as NXF1-NXT1) directly binds the mRNA immediately after splicing and actively participates in mRNA export. ALY/REF, TAP-p15 and UAP56 associate with exon junction complexes (EJC), which are deposited in a splicingdependent manner at 20-24 nt of every exon-exon junction [184, 185] . This interaction network makes spliced mRNAs more susceptible to export and couple splicing and mRNAtrafficking. In fact, unspliced mRNAs are exported by an alternative and less efficient pathway that involves CRM1 and RanGTP. This alternative route is also followed by mRNAs encoding some protoncogenes and cytokines [186, 187] . It is important to mention that although mRNA can follow different nuclear export pathways, in all cases the interaction of export receptors with nucleoporins plays an essential role in the transport of the mRNPs throughout the NPC [170] .