TUESDAY, July 8 (HealthDay News) -- A genetic test to determine whether a breast cancer patient is likely to respond to treatment with the drug Herceptin (trastuzumab) has been approved by the U.S. Food and Drug Administration.

The SPOT-Light HER2 CISH kit helps calculate how many copies of the HER2 gene, which regulates the growth of cancer cells, are in tumor tissue. A healthy breast cell should have two copies of the HER2 gene, but patients with breast cancer may have many more. Since the gene signals cells when to grow, divide and make repairs, too many copies may cause cells to grow and divide too rapidly.

The drug Herceptin targets HER2 protein, helping stop the growth of such cancer cells in breast cancer patients overproducing that particular protein.

"When used with other clinical information and laboratory tests, this test can provide health care professionals with additional insight on treatment decisions for patients with breast cancer," Dr. Daniel Schultz, director of the FDA's Center for Devices and Radiological Health, said in an agency news release.

The SPOT-Light test counts HER2 genes through a chemically stained sample of removed tumor observed under a standard microscope. Previous tests required more expensive and complex fluorescent microscopes. The SPOT-Light also allows labs to store the tissue for later reference, a feature not possible with previously available tests.

The Invitrogen Corp., of Carlsbad, Calif., produces SPOT-Light. San Francisco-based Genentech manufactures Herceptin.

More information

The National Cancer Institute has more about Herceptin.