Genetic lesions in a cell can occur at any age, including the time when a fetus is forming in its mother's womb. Some genetic lesions can be inherited from parents. In other words, some people start life with DNA that has been damaged in some way. Such inherited lesions are present in the fertilized human egg, the single cell from which the new person will grow. In contrast to an inherited lesion, which affects every cell, a person can also develop a genetic lesion during childhood or as an adult.
Cancer is thought to develop after a cell has accumulated several cancer-causing lesions.
Medical authorities commonly estimate that 4 to 10 lesions in a cell's DNA are necessary before a cancer can develop. Some of these lesions may be inherited, and others may be acquired at any age after conception. A few people inherit powerful genetic lesions that confer a high chance of getting cancer in a specific organ, with the cancer often occurring at a young age. Such powerful inherited lesions seem to account for 5 to 10 percent of all human cancers. Even these powerful inherited lesions may need additional lesions before they can turn into a cancer.