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Biological markers or Biomarkers are substances used as an indicator of a biologic state. It is a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.

Biomarkers validated by genetic and molecular biology methods can be classified into three types. 1. Type 0 - Natural history markers 2. Type 1 - Drug activity markers and 3. Type II - Surrogate markers

It can be any kind of molecule indicating the existence (past or present) of living organisms. In particular, in the fields of geology and astrobiology biomarkers are also known as biosignatures. The term is also used to describe biological involvement in the generation of petroleum (see Biomarker (petroleum)).

In medicine, a biomarker can be a substance that is introduced in 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.

In medicine, a biomarker can 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. Once a proposed biomarker has been validated, it can be used to diagnose disease risk, presence of disease in an individual, or to tailor treatments for the disease in an individual (choices of drug treatment or administration regimes). In evaluating potential drug therapies, a biomarker may be used as a surrogate for a natural endpoint such as survival or irreversible morbidity. If a treatment alters the biomarker, which has a direct connection to improved health, the biomarker serves as a "surrogate endpoint" for evaluating clinical benefit.

In psychiatric research, a fruitful way of finding genetic causes for diseases such as schizophrenia has been the use of a special kind of biomarker: endophenotype.

In cell biology, a biomarker is a molecule that allows for 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).

A biomarker can also be used to indicate exposure to various environmental substances in epidemiology and toxicology. In these cases, the biomarker may be the external substance itself (e.g. asbestos particles or NNK from tobacco), or a variant of the external substance processed by the body (a metabolite). (See also: Bioindicator.)

In genetics, a biomarker (identified as genetic marker) is a fragment of DNA sequence that causes disease or is associated with susceptibility to disease.

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