Yet such fortunate patients or would-be patients remain a minority. Most people who get breast cancer have no inherited risk factor. And not everyone among those who inherit a cancer gene will get the disease, Amatruda said. There's probably 15 to 20 percent who don't get the cancer. The fact is, only a small fraction of all cancers are caused by genetic mutations that are passed on from parent to child, said Brian Van Ness, head of the University of Minnesota's Cancer Genetics program.
Experts say that in most cases, there is no single culprit.
Scientists believe that most if not all cancers are caused by a string of errant genes, not just one. Take the case of the colon cancer gene discovered by Mayo Clinic geneticist Gloria Petersen and her former colleagues at Johns Hopkins Medical Institutions. Their quest began in the 1980s. The researchers, led by Dr. Bert Vogelstein, screened medical records of patients with links to the disease, assembled family medical histories and drew blood samples for DNA analysis. After years of work, the Johns Hopkins team and other researchers reported in 1991 they had discovered a gene that, when damaged, leads to one form of colon cancer.