Through cell proliferation and cell cycle analysis, co-treatment of curcumin (10 μg/mL) and trastuzumab (10 μg/mL) significantly reduced cell proliferation and induced G2/M arrest in HER2-overexpressed BT-474 and SK-BR-3-hr (a herceptin resistant strain from SK-BR-3) breast cancer cells, compared to trastuzumab alone. The gene discussed is ERBB2; the disease is breast cancer.