Nonsense mediated decay(NMD) is a mRNA surveillance mechanism to degrade aberrant transcripts that contain premature termination codons (PTCs). A premature termination codon is defined as a STOP codon located more than 50-55bp upstream from the last exon-exon junction. PTCs can be recognized by NMD, resulting in the degradation of the transcripts.
Steven Brenner and colleagues made the first genome-wide analysis on the relationship of pre-mRNA splicing and NMD. They found that alternatively spliced transcript isoforms frequently contained PTCs.
Evidence for the widespread coupling of alternative splicing and nonsense-mediated mRNA decay in humans. Lewis BP, Green RE, Brenner SE. Proc Natl Acad Sci U S A. 2003 Jan 7;100(1):189-92. Epub 2002 Dec 26.
This analysis showed that many alternatively spliced forms might be the result of aberrant splicing which will be cleared by NMD. Beyond that, Brenner and colleagues also advanced the interesting hypothesis that the coupling of alternative splicing and NMD might constitute a functional regulation of gene expression, i.e. using splicing and NMD to control the amount of productive transcripts being made. This hypothesis is supported by several lines of experimental evidence, such as the alternative splicing of PTB.
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http://www.bcm.edu/pathology/cooper/files/Alternative_splicing.htm