It is defined by the U.S. National Library of Medicine's "IUPAC Glossary of Terms Used in Toxicology"[1] as the "Supernatant fraction obtained from an organ (usually liver) homogenate by centrifuging at 9000 g for 20 minutes in a suitable medium; this fraction contains cytosol and microsomes."
The microsomes component of the S9 fraction contain cytochrome P450 isoforms (phase I metabolism) and other enzyme activities.
The cytosolic portion contains the major part of the activities of transferases (phase II metabolism).
[3] The S9 fraction has been used in conjunction with the Ames test[4] to assess the mutagenic potential of chemical compounds.
[6] This article incorporates text from the United States National Library of Medicine, which is in the public domain.