Candidates for APBI can be divided into three groups: “suitable”, “cautionary”, and “unsuitable”, depending on the histological tumor type, diameter, ductal carcinoma in situ (DCIS) presence, excision margin, estrogen receptor (ER) status, lymphatic vessel invasion (LVI), lymph node status, age, and BRCA1 status. Here, ESR1 is linked to ductal breast carcinoma in situ.