design: This study investigated the heart miRNA expression profiles in doxorubicin (DOX)-treated female BALB/c mice bearing 4T1 breast cancer. Eight-week-old female mice were inoculated with 4T1 breast cancer cells and treated with intratumoral injection of either Rab27a-targeting shRNA lentivirus (shRab27a) or control shRNA lentivirus (shNC). DOX was administered at 5 mg/kg weekly. The analysis aimed to understand the role of Rab27a in modulating circulating miRNA patterns during DOX-induced cardiotoxicity in breast cancer.