1. A multimolecular complex composed of 3′–5′-exoribonucleases involved in various RNA surveillance, processing, and degradation pathways. A significant proportion of nuclear exosomes are present in larger complexes (60–80S) whereas cytoplasmic exosomes are all ~10S. Exosomes are found in eukaryotes and archaea.
2. A type of antigen-presenting vesicle secreted by some ‘professional’ antigen-presenting cells. Exosomes are membrane-bounded and enriched in MHC class I and II proteins. Those released by dendritic cells are referred to as dexosomes.
3. In the older literature an exosome was a DNA fragment taken up by a cell that did not become integrated with host DNA but transformed the cell and continued to replicate. This usage is probably redundant.