ERBB2 and breast carcinoma: To evaluate the specificity of this ddPCR assay, we included a set of n = 7 HDs (to define the thresholds on ERBB2 amplification and overexpression) and EV‐RNA extracted from conditioned cultured medium of HER2+ (SKBR3) and HER2‐ (MDAMB231) breast cancer cell lines.