Herceptin, manufactured by Genentech, is given to breast cancer patients whose cancer cells have a lot of copies of the HER2/neu gene. Women who have a lot of copies of this gene usually have cancers that grow and spread more aggressively than other breast cancers. Herceptin has been shown to dramatically slow that growth and help women live longer.

However, the drug is expensive and it can cause side effects. It has been linked to congestive heart failure, especially in women who are taking the drug along with the chemotherapy drugs doxorubicin (Adriamycin) and epirubicin (Ellence).

Two tests are currently used to figure out if a patient's cancer is HER2-positive and will likely respond to Herceptin: fluorescent in situ hybridization (FISH) and immunohistochemistry (IHC). The FISH test counts copies of the HER2 gene, and immunohistochemistry identifies the amount of HER2 receptor protein.

The new test measures the number of copies of the HER2 gene in a patient's tumor tissue, similar to FISH. But unlike FISH, which is expensive and requires specimens be shipped to a lab that has the equipment to view fluorescent probes, SPOT-Light uses a stain that can be viewed with standard microscopes. Unlike FISH, this kit allows labs to store the patient's tumor sample at the lab for future reference, cutting down on the number of repeat tests. IHC-stained slides can also be looked at later, but some doctors feel the test isn't quite as accurate.

The kit should be available in mid-August, according to manufacturer Invitrogen Corp.

HER2-positive cancers can also be treated with lapatinib (Tykerb). For more information on these drugs and the HER-2 gene, see "Breast Cancer: Targeted Therapy" and "How Is Breast Cancer Diagnosed?"