The test is being developed at Llanelli's Prince Phillip Hospital with the help of Singleton Oncology and the University of Swansea.
During recent trials, nearly half of patients who were initially recommended for chemotherapy and endocrine therapy, had their treatment changed to endocrine therapy alone.
This test will have a major impact on the lives of many women, allowing them to get on with their lives following breast cancer surgery rather than having to go through four months of chemotherapy."
The 'Oncotype DX' test, which is only for women with early breast cancer, will give clinicians a clearer idea of the risk and benefit of chemotherapy.
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Tributes have been paid to an ambulance driver killed in a crash with another ambulance in North Wales.
Tonight is expected to be clear and dry - though some parts of Wales will experience frost.
The RSPCA said the animal had found himself in "quite a predicament" after becoming trapped in the tight space.