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Purpose The 21-gene (Oncotype DX) recurrence score (RS) assay is useful in predicting the benefits of adjuvant chemotherapy for early breast cancer patients and is widely used in Western countries. However, to date, it has not gained much popularity in East Asia. We analyzed the results from five institutions' experience from using the 21-gene assay and examined the impact of assay results on decision making of chemotherapy in Korean breast cancer patients and the associations between RS and clinicopathologic characteristics. Materials and Methods The 21-gene assay was performed on 212 patients with estrogen receptor-positive early breast cancer in five institutions. Each center made systemic treatment decisions both before and after the knowledge of assay results. Results Among the 212 patients, 132 (62.3%) had a low RS of < 18, 60 (28.3%) had an intermediate RS of 18-30, and 20 (9.4%) had a high RS of ⩾ 31. Histologic grade, presence of micrometastases, Ki-67, and presence of lymphatic invasion were statistically associated with the RS results. Treatment decisions were changed in 115 of 212 patients (54.2%) in 109 of 212 (51.4%) from chemotherapy plus hormone therapy to hormone therapy, and in six of 212 (2.8%) from hormone therapy to chemotherapy plus hormone therapy. Conclusion The 21-gene breast cancer assay proved to have a significant impact on treatment decision- making. The test reduces chemotherapy use in more than 50% of Korean estrogen receptor-positive, early breast cancer patients. [ABSTRACT FROM AUTHOR]