We summarized essential characteristics of breast cancer patients with tumor size ≤ 2 cm in the training cohort by distant metastasis-free survival (DMFS) time, age, tumor grade, lymph node status, biomarker status (ER, PR, and HER2), and the Prediction Analysis of Microarray 50 (PAM50) results (Table 1). Here, ESR1 is linked to breast cancer.