BRCA1/2 germline mutations occur at about 1/400 in the population at large. In breast cancer patients they occur at about 1/50; in breast cancer patients under

BRCA1/2 germline mutations occur at about 1/400 in the population at large. In breast cancer patients they occur at about 1/50; in breast cancer patients under 40 they occur at about 1/10. I.e., 98% of breast cancer patients and 90% of breast cancer patients under 40 do not have cancer because of this genetic susceptibility. This is by far the most common germline cancer mutation, and it is a severe, DNA damage repair mutation. There are few comparable mutations that will indicate such a severe risk. If you're NF1 mutant or have Li-Fraumeni (other germline cancer-causing mutations) you will have other disease syndromes before you develop cancer. In the vast majority of cases, testing cancer susceptibility genetically will either (1) miss the largest non-genetic component of cancer susceptibility and (2) incorrectly indicate risk (i.e., a positive test does not mean as much).