The recommendation is based on a study that suggests at least 1 million U.S. women carry a gene that makes them ultra-sensitive to X-rays and other radiation sources. About 1 in 5 women with breast cancer carries the gene.

[Michael Swift] says the gene ordinarily causes a rare nerve disorder called ataxia- telangiectasia, or A-T, in people who inherit a copy from both parents. About 1 in 100 people, those with only one copy, are carriers.