nonsense-mediated decay

Defusing Dangerous Mutations: Scientists Discover A New Way By Which Cells Control Genetic Errors: Adapted: "Nonsense-Mediated Decay (NMD), is a process by which cells destroy potentially harmful molecules. Both healthy and damaged proteins begin as instructions in genes. Cells transcribe this information and create an mRNA molecule, a template that will be used to create proteins. RNAs usually contain extra bits of code that have to be cut out before they can be used. During this cut-and-paste operation, cells attach a group of molecules called the exon junction complex (EJC) to the RNA. An RNA made from a mutant gene usually has an EJC in the wrong position, which activates NMD and destroys the RNA before it can be used to make flawed proteins. There are at least two kinds of NMD: one requires UPF2 and the other does not.
The presence or absence of UPF2 changes the composition of the EJC, giving it different surfaces to which other molecules attach. This affects the way that another component, called UPF1, fits onto the machine. UPF1 is directly responsible for calling up the NMD machinery. The study shows that UPF1 can be mounted on both EJC types; the final effect is the same – to efficiently destroy faulty RNAs. "