NEW YORK (Associated Press) - The Food and Drug Administration on Tuesday approved a new genetic test from Invitrogen Corp. which identifies breast cancer patients who will respond to a powerful cancer-fighting drug.

The test from Carlsbad, Calif.-based Invitrogen measures the presence of a cancer-promoting gene to determine whether patients are good candidates for Herceptin, which is made by Genentech Inc.

Herceptin was the first of a new class of cancer drugs which destroy cancer cells without also killing normal cells. It works by targeting the HER-2 gene, which encourages tumor growth. However, only about 20 percent of breast cancer patients have cancer caused by the gene.

The Spotlight HER2 CISH test from Invitrogen identifies which patients are likely to respond to Herceptin based on the amount of HER-2 gene in their tumors.

The Spotlight test differs from previously approved tests because it allows lab technicians to store tumor tissue samples for future reference, according to a statement from the FDA.

Herceptin generated sales of $1.3 billion last year for South San Francisco-based Genentech.