The investigators hypothesized that the GenesWell BCT may help inform decision about whether or not to have adjuvant chemotherapy to patients with high-risk pN0-N1, ER+/HER2- breast cancer in Korea. While adjuvant therapy for ER-positive EBC is effective in reducing risk of recurrence and improving survival, recurrences are still common, especially in patients with unfavorable factors in terms of clinical, pathological and/or molecular perspectives.

Since the addition of CDK4/6 inhibitor, palbociclib, to endocrine therapy (ET) has proven clinical efficacy with tolerable toxicity profile in ER-positive, HER2-negative advanced BC, its use in the adjuvant setting may decrease risk of recurrences in patients with ER-positive, HER2-negative EBC after surgical resection of the primary tumor by enhancing primary endocrine responsiveness and preventing, or delaying the development of acquired resistance for endocrine therapy.

The purpose of this study is to evaluate the effect of addition of palbociclib to standard adjuvant ET on event-free survival (EFS) in patients with ER-positive, HER2-negative EBC but unfavorable clinicopathological (clinical high risk, C-high) and molecular features (genomic high risk, G-high).