Genetics - null mutation

http://www.wormbook.org/chapters/www_epistasis.2/epistasis.html

a null mutation, a loss-of-function mutation, no functional transcript, no function at all

or a gain-of-function mutation

http://en.wikipedia.org/wiki/Nonsense_mutation
Many organisms—including humans and lower species, such as yeast -- employ a nonsense-mediated mRNA decay (NMD) pathway, which degrades mRNAs containing nonsense mutations before they are translated into nonfunctional polypeptides.

(also called an mRNA-surveillance pathway)

http://www.urmc.rochester.edu/labs/Maquat-Lab/
http://www.urmc.rochester.edu/labs/Maquat-Lab/projects/nonsense-mediated_mrna_decay_nmd

http://www.youtube.com/watch?v=oBg6Wpab81M

http://www.answers.com/topic/mrna-surveillance
the process occurring in the cytosol of eukaryotic cells whereby mRNA that contains a premature stop codon is detected and submitted to selective degradation by nonsense-mediated mRNA decay. It involves assembly of a surveillance complex on the mRNA about 24 nt upstream of exon-exon junctions. If translation terminates because of an upstream stop codon, translation release factors bind to the surveillance complex and trigger mRNA degradation.