Sometimes, even when a suspected cluster has enough cases for study, a true statistical excess cannot be demonstrated. Other times, epidemiologists find a true excess of cases, but they cannot find an explanation for it. For example, the suspected carcinogen may cause cancer only under certain circumstances, making its impact difficult to detect. Moreover, because today's populations move so often, it can be difficult for epidemiologists to identify previous exposures and find old records. Researchers are just beginning to unravel the puzzle of carcinogenesis in terms of the roles of heredity and environmental exposures.
All cancers develop because of genetic mutations.
Some mutations that increase the risk of cancer are present at birth in the genes of all cells in the body. If a mutation is present in all cells, including reproductive cells, it can be passed from parent to child. This is known as an inherited susceptibility. Most cancers are not due to an inherited susceptibility but result from genetic changes that occur during one's lifetime within the cells of a particular organ. Because cancer is a common disease, it is not unusual for several cases to occur within a family.