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A biomarker, or biological marker, is an indicator of a biological state, or the past or present existence of a particular type of organism. Biomarkers are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. They are used in many scientific fields. The presence and concentration of certain molecules, or measured activity such as electrical activity in the brain, provide biomarkers that can be identified and measured. Some of the more colorful and illustrative terms used to describe molecular biomarkers are "signature molecules" and "fingerprint molecules."
In medicine, a biomarker can be a traceable substance that is introduced into an organism as a means to examine organ function or other aspects of health. For example, rubidium chloride is used as a radioactive isotope to evaluate perfusion of heart muscle. It can also be a substance whose detection indicates a particular disease state, for example, the presence of an antibody may indicate an infection. More specifically, a biomarker indicates a change in expression or state of a protein that correlates with the risk or progression of a disease, or with the susceptibility of the disease to a given treatment.
Biochemical biomarkers are often used in clinical trials, where they are derived from bodily fluids that are easily available to the early phase researchers. A useful way of finding genetic causes of diseases such as schizophrenia has been the use of a special kind of biomarker called an endophenotype.
Other biomarkers can be based on measures of the electrical activity of the brain (using Electroencephalography or Magnetoencephalography), or volumetric measures of certain brain regions (using Magnetic resonance imaging )
 Cell biology
In cell biology, a biomarker is a molecule that allows the detection and isolation of a particular cell type (for example, the protein Oct-4 is used as a biomarker to identify embryonic stem cells).
In genetics, a biomarker (identified as genetic marker) is a DNA sequence that causes disease or is associated with susceptibility to disease. They can be used to create genetic maps of whatever organism is being studied.
 Geology and astrobiology
A biomarker can be any kind of molecule indicating the existence, past or present, of living organisms. In the fields of geology and astrobiology, biomarkers, versus geomarkers, are also known as biosignatures. The term biomarker is also used to describe biological involvement in the generation of petroleum.
Biomarkers are used to indicate an exposure to or the effect of xenobiotics which are present in the environment and in organisms. The biomarker may be an external substance itself (e.g. asbestos particles or NNK from tobacco), or a variant of the external substance processed by the body (a metabolite) that usually can be quantified. (See also: Bioindicator.)
 See also
- Biomarker discovery
- Biomarkers (journal)
- Biomarker insights - a journal
- Molecular marker
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