To evaluate whether the increased cardiac miR-338-3p is of breast cancer origin and transmitted to recipient heart/cardiomyocytes in a sEV-dependent manner, we first detected the levels of miR-338-3p in orthotopic breast cancer tumor bearing mice and non-tumor-bearing mice, the results showed that miR-338-3p was consistently up-regulated by DOX in plasma, plasma sEVs and cardiac tissues of tumor bearing mice but not in non-tumor-bearing mice or tumor bearing mice subjected to intratumoral injection of lentiviral shRNA targeting Rab27a (Supplementary Fig. 5A–C). This evidence concerns the gene RAB27A and breast cancer.