Background: The most important and well-established benefit of neoadjuvant therapy for breast cancer patients is increased breast conservation rate. However, in luminal A type breast cancer, the response to neoadjuvant chemotherapy (NCT) is not as good as other subtype of breast cancer, such as HER-2 or triple-negative breast cancer. In addition, with the advancement of multi gene assay tools for this subtype, adjuvant chemotherapy is not needed at all in significant proportion of this luminal A subtype. Through a selective neoadjuvant therapy, either chemotherapy or endocrine therapy using histopathologic markers and 70-gene assay (Mammaprint. Agendia inc.), we hypothesize that we could increase the breast conservation rate in lumnal A type breast cancer. Trial design: This study is a non-randomized, phase II, prospective study. The main inclusion criteria is women with stage I-IIIA, ER-positive, HER-2 negative breast cancer that tumor size is measurable. Breast conserving surgery (BCS) is not feasible considering the turmor size, location, and patient's breast size. Two surgeons in each institution will judge the feasibility of BCS. Main exclusion criteria is diffuse malignant microcalcifiation or multicentrica breast cancer. The conversion rate from BCS-ineligible to BCS-eligible with NCT was 35.8% in luminal A breast cancer in our previous study. We assumed that with the study regimen, the rate will be increased to 50.8% (15% increase). Given these estimates, under 10% type I error rate and 70% power, 220 patients in total will be enrolled from nine tertiary hospitals in Korea. All the patients initially will be tested with Mammaprint assay. When the Mammprint result is high risk, the patients will receive NCT. When the Mammprint result is low risk, the patients will receive neoadjuvant endocrine therapy. Postmenopausal women receive letrozole 2.5mg per day for 16 weeks. Premenopausal women receive leuprorelin every 4 weeks with letrozole for 16 weeks. The primary endpoint is conversion rate from BCS-ineligible to BCS-eligible of more than 50%. The secondary endpoint is actual breast conservation rate. pathologic complete response. clinical response rate, and disease-free survival. Legal entity responsible for the study: Korean Breast Cancer Study Group (KBCSG). Funding: Agendia Takeda Kwandong Pharmaceutical Co Ltd. Korea Shin Poong Pharm, Co. Ltd. Disclosure: The author has declared no conflicts of interest.