A biological molecule found in blood, other body fluids, or tissues that is a sign of a normal or abnormal process, or of a condition or disease. A biomarker may be used to see how well the body responds to a treatment for a disease or condition. It is also called molecular and signature molecule. In cancer research and medicine, biomarkers are used in three primary ways: to help diagnose conditions, as in the case of identifying early stage cancers (Diagnostic); to forecast how aggressive a condition is, as in the case of determining a patient's ability to fare in the absence of treatment (Prognostic); to predict how well a patient will respond to treatment (Predictive).

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