A Biomarker is defined as anything that can be used as an indicator of a particular disease state or some other biological state of an organism. A biomarker can be a 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. Biomarkers can be specific cells, molecules, or genes, gene products, enzymes, or hormones.