Researchers in Boston have found that a mutant gene is responsible for about 20 percent of all cases of breast cancer among Jewish women who develop the disease by the age of 40.

They also have determined that women with the genetic defect have 20 times the risk of developing breast cancer by the age of 40 than those who don't have it.

"Until we have more definitive data, all this ought to be observed with caution," said Dr. Rosemary Duda of Beth Israel Hospital, a co-author. "It shouldn't set off panic."