A cancer-causing substance or agent. There are three major stages in cancer development.
Initiation, in which a cancer-causing agent first damages a cell's genetic material.
Promotion, when cells in which initiation has occurred are exposed to chemicals that speed up cell division. Such chemicals cannot initiate cancer themselves: Generally long-term exposure to these promoters is necessary for cancer to develop. Nutritional factors are thought to have their greatest impact on cancer in this stage.
Progression is the final stage of cancer development when cells become fully malignant and acquire the ability to metastasize, or spread, to other parts of the body.