There are several defnitions of biomarkers. A biomarker is a characteristic that can be objectively measured and evaluated as an indicator of a physiological as well as a pathological process or pharmacological response to a therapeutic intervention. Classical biomarkers are measurable alterations in blood pressure, blood lactate levels following exercise and blood glucose in diabetes mellitus. Any specifc molecular alteration of a cell on DNA, RNA, metabolite or protein level can be referred to as a molecular biomarker. In the era of molecular biology, biomarkers usually mean molecular biomarkers and can be divided into three broad categories: 1. Those that track disease progression over time and correlate with known clinical measures 2. Those that detect the effect of a drug 3. Those that serve as surrogate endpoints in clinical trials While researchers are studying all three categories, biotechnology and pharmaceutical companies favor using biomarkers as drug discovery tools – not only to detect biological responses to experimental drugs but also to aid in the discovery of new targets for therapeutic intervention. A biomarker can be as simple as a laboratory test or as complex as a pattern of genes or proteins. From a practical point of view, the biomarker would specifcally and sensitively reflect a disease state and could be used for diagnosis as well as for disease monitoring during and following therapy. The term “negative biomarker” is used for a marker that is defcient or absent in a disease.