Everyone in the population is at risk of developing cancer at some point in their lives. Not everyone who has cancer will necessarily have inherited a faulty copy of a susceptibility gene.

In the general population (where there is not an inherited faulty susceptibility gene that causes a predisposition to cancer), cancer develops as a result of a cascade of genetic changes to a single cell and its descendants leading to uncontrolled growth in a subset of cells in the body. Because the genetic mistakes are accumulated over time, the risk of developing cancer increases with age. In an individual who has a faulty cancer susceptibility gene present, the faulty gene is usually present in every cell in the individual (including the embryo).

This means that fewer genetic mistakes are needed to accumulate in an individual cell of that person for cancer to develop, since the cell already contains one mistake. Because less time is needed to accumulate the mistakes, cancer often develops earlier in people who inherit a faulty susceptibility gene than in people with the sporadic cancer.

For example, women that have an inherited fault in the gene that causes breast cancer are more likely to develop it in their late thirties and forties, whereas women that develop the cancer sporadically are more likely to get it in their sixties and seventies.

These inherited cancer susceptibility conditions are responsible for only a small proportion of cancers in the population.

About 5% of individuals with common cancers have an inherited cancer susceptibility condition. The risk of developing cancer that is associated with a susceptibility gene condition can vary depending on the specific condition. Some genetic faults can cause a very high susceptibility to specific cancers, such that almost all people who have the faulty gene will develop the condition. These conditions are called highly penetrant conditions. For other conditions, having an inherited copy of the faulty susceptibility gene only results in the cancer developing in a proportion of individuals that carry the susceptibility gene.

These conditions will be referred to as lower penetrance susceptibility conditions.