As stated above, the exosome complex contains many proteins with ribonuclease domains. The exact nature of these ribonuclease domains has changed across evolution from bacterial to archaeal to eukaryotic complexes as various activities have been gained and lost. The exosome is primarily a 3'-5' exoribonuclease, meaning that it degrades RNA molecules from their 3' end. Exoribonucleases contained in exosome complexes are either phosphorolytic (the RNase PH-like proteins) or, in eukaryotes, hydrolytic (the RNase R and RNase D domain proteins). The phosphorolytic enzymes use inorganic phosphate to cleave the phosphodiester bonds – releasing nucleotide diphosphates. The hydrolytic enzymes use water to hydrolyse these bonds – releasing nucleotide monophosphates.