A study published today in the journal Nature Genetics found that one percent of American Jewish women of Ashkenazi descent carry a mutation of the gene, BRCA1, associated with breast cancer. The discovery has great promise of allowing scientis ts to gain a better understanding of the incidence of breast cancer. Women with a family history of breast cancer and the mutated gene, 185delAG, have an eighty to ninety percent chance of getting the disease. They also have a forty to fifty percent chan ce of getting colon cancer. A new study is beginning immediately to determine exactly how much of a cancer risk the mutation signifies for Jewish women without a history of breast cancer in their family. Unfortunately, no known cure exists for cancer and genetic testing raises questions of what therapy to provide for persons with positive results.