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Historically, in selecting adjuvant chemotherapy for patients with breast cancer, anatomy, including tumor size and nodal status, has played the primary role. As a result of analyses of genomic and clinical data, breast cancers are now thought to be a family of diseases. Major subtypes of breast cancer include HER-2 positive disease, basal-like or triple negative tumors, and at least two types of hormonally sensitive cancers. Using the nomenclature developed in the gene expression array studies, these two types are often referred to as luminal A and luminal B. Estrogen receptor negative tumors relapse earlier than estrogen receptor positive tumors. In general, estrogen receptor negative cancers are also more responsive to chemotherapy. In contrast, estrogen positive tumors appear to be somewhat less responsive to chemotherapy, but endocrine therapy can be of substantial benefit in decreasing the risk of disease recurrence. Women with HER-2 positive disease derive significant benefit with the use of trastuzumab. The role of chemotherapy in preventing disease recurrence in patients with estrogen receptor positive tumors is being reevaluated. Recent data suggests that chemotherapy has the greatest benefit in those patients with estrogen receptor positive cancers whose tumors are HER-2 positive disease, weakly ER positive, and/or have high nuclear grade. In the future, breast cancer treatment will be more targeted to the tumor and tailored to the individual with the use of genomic and clinical data.