Gene signatures may complement clinical and pathological factors to predict prognosis and response to therapy in patients with breast cancer, and can also sub-classify these tumours into entities with different biology and treatment requirements. A number of prognostic gene signatures are commercially available at this moment and two of them have entered phase III evaluation. Specific signatures are also being assessed to predict response to a number of drug therapies. The combined use of prognostic, predictive and subtype-defining signatures will guide therapeutic decisions in the future and will facilitate development of targeted drugs in specific groups of patients. However, cost-utility issues and some technical limitations have hindered widespread adoption of gene profiling. Gene signatures will become part of the routine clinical workup only if they help making clinical decisions. The first step to achieve this will consist of the inclusion of gene signatures in the design of clinical trials with new drugs.
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