Prior to export from the nucleus of the eukaryotic cell, pre-mRNA must be modified, since it contains one or more non-amino acid coding sequences called introns.  These must be excised from the portions of the molecule called exons, which do carry amino acid coding sequences.


RNA splicing can serve as a mechanism of regulatory control by:

1.  excising introns at different rates, thus speeding or slowing the rate of protein
     synthesis in the cytoplasm.

2.  producing different proteins (isoforms which have similar functions, but different
     structures) by alternative splicing- exicing some exons and including others
     during the modification of the pre-mRNA transcript.

Degradation of pre-mRNA can also serve a regulatory function.  Only a fraction of the transcripts produced in the nucleus actually enter the cytoplasm as mature mRNA.  The rest, either made at the wrong time, or by an innappropriate cell, are degraded in the nucleus.