We may conclude, from all of these considerations, that the burden of cancer falls unequally upon a population of individuals. Some normal persons develop cancer as a result of bad luck; they are victims of spontaneous processes or background processes such as gene mutation or chromosomal rearrangement. Others are susceptible because they are exposed to environmental agents that increase these background rates. But there are individuals who are predisposed to cancer because they carry mutations that are already on the path to cancer or carry genes that increase the prospect that such mutation will occur, either spontaneously or in response to an environmental agent. It is even possible that most environmental cancer also involves genetic predisposition.