An isotope of artificial or natural origin that exhibits radioactivity.Radionuclides serve as agents in nuclear medicine and genetic engineering, play a role in computer imaging for diagnosis and experiment, and account for a percentage of background radiation to which humans are exposed. In cancer therapy, radionuclides that localise to certain organs (e.g., radioactive iodine or gallium), deliver cytotoxic radiation doses to tumours. Similarly, radionuclides can be yoked to monoclonal antibodies engineered to attack specific populations of cancerous cells. In positron emission tomography, glucose molecules tagged with radionuclides are injected into the bloodstream. The gamma radiation emitted by the decay of the radionuclides reveals areas of active glucose uptake and thus offers a gauge of cell metabolism and function.