5 to 20% of cancers arise in families that show a very strong, hereditary susceptibility to a particular type of cancer. The cancer-susceptibility in these families is usually highly penetrant, in other words any individual who inherits the faulty gene has a strong likelihood of developing cancer. Affected individuals in these families carry a germ-line (i.e. inherited) mutation - which is therefore present in every cell of their body - that makes it much more likely that their somatic cells will become cancerous.

Genetic testing for hereditary-cancer mutations

Genetic tests have been developed to detect the rare mutations known to be implicated in some types of hereditary cancer, such as inherited breast and bowel cancer. If a mutation is found in an affected family member, it is possible to test other family members to determine whether they have inherited the cancer-causing mutation and thus whether they are likely to develop disease. Genetic testing for hereditary-cancer mutations is only of use where there are strong indications from family history or clinical features that a such a mutation is likely to be present in the family. It is not appropriate in the majority of patients with a family history of cancer, as only a small number of such families carry mutations in known hereditary-cancer genes .