Brouwer A, De Laere B, van Dam PJ, et al. - Given that accurate assessment of molecular aberrations, eg, overexpression of human epidermal growth factor receptor 2 (HER-2), form the basis of personalized targeted treatment in metastatic breast cancer (MBC) and that for real-time biomarker assessment, molecular interrogation of FDA-cleared CellSearch-enriched circulating tumor cells (CTCs) can provide an attractive alternative, researchers examined how the immunofluorescent (IF) described HER-2 categories reflect gene expression and amplification, and if consideration could be given to ‘medium’ HER-2 expressing CTCs for patient selection. Using CellSearch, they enriched tumor cells from spiked cell lines (n = 8) and CTCs (n = 116 samples) of 85 metastatic breast cancer patients. Using IF imaging (ACCEPT, DEPArray, and visual scoring) with quantitative reverse transcription(qRT)-PCR and HER-2/neu fluorescent in-situ hybridization, they performed comparative analysis of HER-2 expression. Findings suggest that in patient samples containing ≥ 5 CTCs, HER-2 characterization on CTCs should be focused on HER-2^high expressing CTCs. Although patients display heterogeneous CTC-derived HER-2 expression, the incidence of HER-2-positive MBC seen in the clinic is better indicated by the prevalence of patients with ≥ 1 HER-2^high expressing CTCs. Furthermore, this work established that only HER-2^high expressing tumor cells harbor amplification of the HER-2/neu gene, and samples containing HER-2^high expressing CTCs show high relative gene expression of HER-2 on qRT-PCR.