Describes how likely it is that a person who has a certain disease-causing mutation (change) in a gene will show signs and symptoms of the disease. Not everyone who has the mutation will develop the disease. For example, some people who have a BRCA1 or BRCA2 gene mutation will develop cancer during their lifetime, but others will not. Currently, there is no way to know which people who have a cancer-causing mutation will develop cancer. Complete penetrance means that every person who has the mutation will show signs and symptoms of the disease.