Estrogen receptor (ER) and progesterone receptor (PgR) status determines the treatment options available to breast cancer patients. Since the measurement of these proteins was first described, there have been challenges to their overall accuracy. Even today, in the age of molecular tools and automation, there is still a surprisingly high rate of misclassification and, consequently, a negative impact on patient treatment and outcome. Here we examine this problem and suggest a solution. Breast cancer is one of the most common cancers among women worldwide. Surgery, radiotherapy and sometimes chemotherapy are the optimal treatment options for patients diagnosed with this disease. In addition, adjuvant therapy is available to patients with tumors expressing the ER and/or the PgR. Approximately 70-75% of breast cancers express ER and/or PgR, thus they are eligible for endocrine therapy (also referred to as estrogen-receptor modulators), using drugs such as tamoxifen, aromatase inhibitors and gona-dotropin-releasing hormone agonists [1]. While only a percentage of ER/PgR-positive patients will respond to therapy, ER/PgR-negative cases do not respond. Here we examine the current status of this companion diagnostic test.
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