Introduction to Gene Expression Profiling in Breast Cancer

When Halsted introduced the radical mastectomy, there was no formal method for estimating survival after tumor resection. Formal TNM categorization later refined disease staging and facilitated estimates of prognosis, but linked outcome conceptually with tumor burden, rather than tumor biology. New molecular assays based on gene expression profiling of the lesion have emerged. These assays are providing a fundamentally new mechanism for breast cancer classification, yielding valuable new insight into prognosis beyond TNM staging, and appear to provide enhanced guidance regarding choice of therapy for at least some breast malignancies. Randomized clinical trials are currently underway to assess the accuracy of these assays versus existing clinic-pathologic guides. A fundamental understanding of these emerging profiling tools is necessary for breast specialists.