The core of the exosome contains a six-membered ring structure to which other proteins are attached. In eukaryotic cells, the exosome complex is present in the cytoplasm, nucleus, and especially the nucleolus, although different proteins interact with the exosome complex in these compartments regulating the RNA degradation activity of the complex to substrates specific to these cell compartments. Substrates of the exosome include messenger RNA, ribosomal RNA, and many species of small RNAs. The exosome has an exoribonucleolytic function, meaning it degrades RNA starting at one end (the 3′ end in this case), and in eukaryotes also an endoribonucleolytic function, meaning it cleaves RNA at sites within the molecule.