SK-BR-3 cells had relatively higher CI values than BT-474 with CI ranging from 0.12 ± 0.14, 0.60, ± 0.68, and 0.4 ± 0.6, and 0.34 ± 0.46 at 100, 500, 1000, and 10,000 nM, respectively (Fig. 4 h), indicating reduced synergy against breast cancer cells with lower HER2 overexpression on the cell membrane than BT-474 cells66. Here, ERBB2 is linked to breast cancer.