Since trastuzumab was mainly applied to treat patients with breast cancer that showed high expression of the HER2 gene, we performed an in vitro study to compare the difference in anti-tumor effects between anti-HER2 IgG-abundant plasma and trastuzumab on two breast cancer-derived cell lines, SK-BR-3 and BT-474, which highly express HER2 (Supplementary Materials and Methods). Here, ERBB2 is linked to breast cancer.