Two genes, BRCA1 and BRCA2, occur infrequently in the population but are relatively common in women of Ashkenazi Jewish ancestry and in women with a strong family history of breast cancer. When a woman has defects in 1 of these genes, she is very likely to have breast (or ovarian) cancer during her lifetime. Many women are worried that they have mutations of these genes, and they seek advice about whether to be tested for it. Because the mutation test is expensive, counselors estimate the probability that a woman has the BRCA1/BRCA2 mutations. One way to estimate the probability of the mutation is to ask questions and then use the answers to make an educated guess. The accuracy of the guess will depend on the skill and experience of the counselor. However, better ways exist. They are called prediction models. They use knowledge about the exact relationship between a woman's history—including how many family members had breast cancer—and defects in the BRCA1/BRCA2 genes. They obtain this knowledge by asking women about their medical history, testing each woman for mutations of the BRCA1/BRCA2 genes, and doing statistical tests to identify which parts of the history are most strongly related to having the mutations. Using these models, anyone—a counselor or a patient—could calculate a woman's probability of having BRCA1/BRCA2 mutation. Many prediction models exist.