ERBB2 and breast cancer: When SK-BR-3 and BT-474 cells were cultured in medium containing 20% anti-HER2 IgG-abundant plasma only and in medium containing 20% anti-HER2 IgG-deficient plasma and 200 μg/ml trastuzumab, respectively, trastuzumab could significantly inhibit the proliferation of BT-474 cells (p = 1.49 × 10−7) but not SK-BR-3 cells (Supplementary Figures S1A,B), whereas anti-HER2 IgG-abundant plasma could significantly inhibit the proliferation of both breast cancer cell lines (p = 0.006 for SK-BR-3 cells and p = 1.71 × 10−7 for BT-474 cells).