The U.S. regulators approved on Tuesday a genetic test that allows doctors to determine whether a breast cancer patient is likely to respond to treatment with Herceptin (trastuzumab).

Invitrogen's SPOT-Light HER2 CISH kit tracks the number of copies of the HER2 gene in tumor tissue. Herceptin, made by Genentech Inc., and approved in 1998, is specially designed to treat breast cancer patients with high amounts of the HER2 protein.

Normally, a healthy breast cell has only two copies of the HER2 gene. This gene creates excessive amounts of a HER2 protein and causes cancerous cells to grow and duplicate at an extremely high speed. About 20 percent of breast cancer patients have cancer caused by the gene.

According to a report in the New England Journal of Medicine in 2005, cells with high levels of HER2 have also been linked to cancer recurrence after chemotherapy.

The new test counts HER2 genes through a chemically stained sample of removed tumor observed under a standard microscope. Previously, doctors needed more expensive and complex fluorescent microscopes in order to do that. Another benefit of the new test consists in the possibility to store the tissue for later reference, a feature not possible with previously available tests.

“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 Food and Drug Administration’s Center for Devices and Radiological Health, said in a statement, according to Reuters.

According to the American Cancer Society, about 182,000 women will be diagnosed with breast cancer this year and 40,000 will die from the disease.