ERBB2 and breast cancer: Fig 3 shows an example of a BM DTC from a BC patient with amplification of the HER2 gene. Significantly, the primary tumor from this same patient (5324) was scored as negative for HER2 gene amplification. Discordance in HER2 gene amplification between primary tumors and DTCs has been previously documented and has possible treatment implications for targeted therapeutics [29–31]