Breast cancer patients won a breakthrough new treatment late Friday as the Food and Drug Administration approved Herceptin - the first in what promises to be a wave of therapies attacking cancer at its genetic roots.

Herceptin is not for every woman with breast cancer. But for the 30 percent whose especially aggressive tumors are fueled by a bad gene called HER2, adding Herceptin can substantially increase the chances of battling the cancer and living a little longer.Some 50,000 American women a year could benefit from the drug, manufacturer Genentech Inc. estimates.

"For certain women with advanced disease, this new product can mean new hope," said Health and Human Services Secretary Donna Shalala.

The FDA approved Herceptin for advanced breast cancer patients whose HER2-positive tumors have spread to other organs, women who until now have had little hope. They can add it to the drug Taxol as first-line therapy, or use it alone after failing standard chemotherapies.

To make sure the right patients try Herceptin, the FDA also approved a special genetic test kit, HercepTest by Denmark-based Dako Corp., to help doctors measure levels of the bad gene.

Herceptin is a genetically engineered drug that hones in on the gene fueling growth of aggressive cancer.

"The increased used of biological products such as Herceptin to treat the underlying causes of disease is an exciting development in medicine," said FDA Acting Commissioner Michael Friedman, who is a cancer specialist.

The man whose genetic research led to Herceptin considers it an even bigger development: "It heralds a new age in how we're going to treat cancer," said Dr. Dennis Slamon of the Jonsson Cancer Center at the University of California, Los Angeles.

San Francisco-based Genentech said Herceptin would be on pharmacy shelves next month. Women will get weekly intravenous infusions. A price has not been finalized, Genentech said.

Some 30 percent of breast cancer patients have too many copies of a gene called HER2. A healthy version of this gene produces a protein that signals cells to grow and multiply normally. But in women who have too much HER2, the breast cells reproduce out of control and spread through the body.

Genentech genetically engineered an antibody that blocks excess HER2, shrinking and some-times eliminating tumors.

It's not a magic bullet - it helped only half the women who tested it. But tests in very sick women found that adding Herceptin to standard treatment doubled their chances that tumors would shrink. In women who already had failed standard chemotherapy, Herceptin alone cut in half tumors in 15 percent of women.

A handful of women went into remission. Most eventually relapsed, but one is alive an amazing six years later and another three years later.

Unlike chemotherapy, Herceptin caused few serious side effects in clinical testing.