damaged or defective ribosomes and target them for degradation. The surveillance mechanism is in place to detect nonfunctional pre-ribosomes as well as nonfunctional mature ribosomes. In addition, the surveillance system brings the necessary degradation equipment and actually degrades the nonfunctional ribosomes. Pre-ribosomes that build up in the nucleus are destroyed by the exosome, which is a multisubunit complex with exonuclease activity. If defective ribosomal subunits do happen to make it out of the nucleolus and into the cytoplasm, there is a second surveillance system in place there to target malfunctioning ribosomes in the cytoplasm for degradation. Certain mutations in residues of the large ribosome subunit will actually result in RNA decay and thus degradation of the unit. Because the amount of defects that are possible in ribosome assembly are so extensive, it is still unknown as to how the surveillance system detects all defects, but it has been postulated that instead of targeting specific defects, the surveillance system recognizes the consequences of those defects – such as assembly delays. Meaning, if there is a disruption in the assembly or maturation of a mature ribosome, the surveillance system will act as if the subunit is defective. Human disease Mutations in ribosome biogenesis are linked to several human ribosomopathy genetic diseases, including inherited bone marrow failure syndromes, which are characterized by a predisposition to cancer and a reduced number of blood cells. Ribosomal dysregulation may also play a role in muscle wasting. See also RNA polymerase References Ribosome Cellular processes