We tested the sensitivity of four HER2+ breast cancer cell lines (SKBR3, BT474, BT474-Tr, and JIMT1) to trastuzumab by CCK-8, and found that the IC50 values of SKBR3 and BT474 were between 10 and 15 μg/mL, and the IC50 of BT474-Tr was much higher than that of the former. This evidence concerns the gene ERBB2 and breast cancer.