The existence of HER2+ and HER2− subpopulations that can interconvert suggests underlying tumor cell plasticity in these advanced patient-derived breast CTCs, rather than a hierarchical cancer stem-cell model as described in drug-resistant subpopulations within established breast cancer cell lines [59] and may therefore explain the differences between CTC-ITB-01 and MCF7 cells within our experiments. This evidence concerns the gene ERBB2 and cancer.