In the current study, the cytotoxic effects of idarubicin and idarubicin-ZHER2 affibody conjugate on two different HER2-overexpressed cell lines, ovarian malignant cells (SK-OV-3) and breast cancer cells (SK-BR-3), were compared to MCF-7 as a low expressing HER2 cell line. The gene discussed is ERBB2; the disease is breast cancer.