Seven months after the initiation of aromatase inhibitor therapy, when the patient was 69 years old, following the approval of T-DXd for HER2-low breast cancer, cytology of ascitic fluid confirmed HER2-low luminal B disease (ER+, PgR+, HER2 IHC 1+ by VENTANA 4B5, Ki-67 LI 20%–30%) (Fig. 2). This evidence concerns the gene ERBB2 and breast cancer.