ERBB2 and breast cancer: To determine whether reduced HER2 protein expression in TDM1R and TDXdR BC cell lines was due to downregulation of gene expression levels, we performed a droplet digital PCR assay and observed reduction of ERBB2 gene copy numbers in all TDM1R and TDXdR BC cell lines (SUM190-TDM1R and SUM190-TDXdR: > 80% reduction, P < 0.0001, SUM190-TDM1R and SUM190-TDXdR: > 40% reduction, P < 0.001) compared to in parent cell lines (Fig. 2G).