NEW YORK — Researchers say they have discovered a major reason why women who inherit a mutated version of the gene BRCA1 run a high risk of breast cancer — and that finding might aid the search for new treatments.

A second gene, called PTEN, plays a key role, scientists said in a study released Sunday.

Scientists have long known that BRCA1 normally repairs damage to other genes. So if it is crippled by mutations, the unrepaired damage could be expected to lead to cancer. But the specifics of that story have been murky.