It has been reported that spliced transcripts are exported faster from the nucleus to cytoplasm than their unspliced counterparts [5657] indicating the association between splicing machineries and nuclear export, although there are some contradictory studies [5859]. In fact, nuclear transport to the cytoplasm of transcripts containing introns in their 5' UTRs was known to be regulated by the transcription export complex and the serine/arginine-rich (SR) proteins, whereas the transport of transcripts lacking introns in their 5' UTRs was regulated by signal sequences located in the open reading frames (ORFs) of those genes [60]. A recent experiment using fluorescence in situ hybridization has investigated how intron-bearing and intronless constructs are distributed differently across the nucleus and cytoplasm and showed that intron-bearing transcripts are preferentially located in the cytoplasm [31].
