Biomarker

# Overview
A Biomarker is a substance 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.
- Type 0 - Natural history markers
- Type 1 - Drug activity markers and
- 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.

# Medicine
In medicine a biomarker is an indicator of a particular disease state or a particular state of an organism.
An NIH study group committed to the following definition in 1998: "a characteristic that is objectively measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention." 
In the past, biomarkers were primarily physiological indicators such as blood pressure or heart rate.  More recently, biomarker is becoming a synonym for molecular biomarker, such as elevated prostate specific antigen as a molecular biomarker for prostate cancer.  Biomarkers also cover the use of molecular indicators of environmental exposure in epidemiologic studies such as human papilloma virus or certain markers of tobacco exposure such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK).
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.

# Cell Biology
A biomarker can be understood as a molecule that is present (or absent) from a particular cellular type. This facilitates the characterization of a cell type, their identification, and eventually their isolation. Cell sorting techniques are based on cellular biomarkers (for example, Fluorescent-activated cell sorting).
One example of cellular biomarker is the protein Oct-4 that is found in 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.

# Biomarker discovery
Biomarker discovery is the process by which biomarkers are discovered. It is a medical term.
Many commonly used blood tests in medicine are biomarkers. The way that these tests have been found can be seen as biomarker discovery. However, their identification has mostly been a one-at-a time approach. Many of these well-known tests have been identified based on clear biological insight, from physiology or biochemistry. This means that only a few markers at a time have been considered. One example of this way of biomarker discovery is the use of injections of inulin for measuring kidney function. From this, one discovered a naturally occurring molecule, creatinine, that enabled the same measurements to be made easily without injections. This can be seen as a serial process.
The recent interest in biomarker discovery is because new molecular biologic techniques promise to find relevant markers rapidly, without detailed insight into mechanisms of disease. By screening many possible biomolecules at a time, a parallel approach can be tried. Genomics and proteomics are some technologies that are used in this process. Significant technical difficulties remain.
There is considerable interest in biomarker discovery from the pharmaceutical industry. Blood test or other biomarkers could serve as intermediate markers of disease in clinical trials, and also be possible drug targets.