Our preliminary work, in a retrospective cohort of StageIVAggressive (CTCs count ≥ 5 in 7.5 mL) breast cancer patients, demonstrated that the presence of HER2-low CTCs (1+) was correlated with a worse prognosis and a specific, more aggressive, metastatic behavior [11,12]. This evidence concerns the gene ERBB2 and breast carcinoma.