Individuals who inherit cancer susceptibility mutations inherit a predisposition to cancer, not cancer itself. Some mutation carriers inherit their predisposing genotypes in an autosomal dominant fashion, yet they do not develop cancer, indicating that their altered genes are incompletely penetrant. A somatic mutation in a second allele is required for cancer to develop.

Further confusing the situation is the fact that sporadic forms of cancer may also occur in families along with a hereditary cancer syndrome. These cases of sporadic cancer are called phenocopies because their phenotype is similar to that of the affected mutation carriers, but their genotype is different. Genetic testing may determine if the cancer is hereditary or sporadic in nature.