Gene screens are fast becoming a powerful tool, not just for diagnosing cancer but for treating it as well. Joining the growing pool of genetic tests for breast cancer, SPOT-Light mines patients' genes to determine who will respond best to the cancer drug Herceptin, which is effective against tumors that release an abundance of the HER2 protein. The SPOT-Light test can measure how many HER2 genes are present in a sample of breast tumor; the more genes there are, the more likely the tumor will respond to treatment with Herceptin. Breast cancer patients are also increasingly relying o n another gene test, OncotypeDx, which can determine the risk of breast cancer recurrence and which chemotherapy agents will work best against a particular tumor.