Researchers led by Dr. Thomas Mack of the University of Southern California School of Medicine have reported that women who have an identical twin with breast cancer have a four-times greater chance of developing the disease themselves.

"Doctors could use this information to identify women who are particularly susceptible to breast cancer and advise them accordingly," said Mack.

Mack's team compared the rate of breast cancer among the general population and 2,562 pairs of identical twins and non-identical twins with either one or two cases of the disease. They knew that a strong family history of breast cancer was a significant risk factor, but they were surprised by the significance of the identical twin link.