在癌症肿瘤转移中，血液中的单个循环肿瘤细胞（CTCs）和骨髓中散布的肿瘤细胞（DTCs）介导癌症转移。由于缺乏合适的生物标志蛋白，难以从正常血细胞中分离出具有间充质属性的CTCs和DTCs。为了建立一种允许分离这些细胞的程序，我们通过细胞培养中稳定同位素标记（SILAC）和质谱分析，分析了从乳腺癌患者的骨髓DTCs建立的BC-M1细胞系。我们发现具有间充质属性的乳腺癌细胞系中CUB结构域含蛋白1（CDCP1）的转膜蛋白水平很高。外周血单个核细胞对CDCP1几乎为阴性。通过CellSearch在30名分析的乳腺癌患者中发现8例CDCP1阳性CTCs。只有EpCam阳性的CTCs被CellSearch富集。使用CDCP1的细胞外结构域，我们建立了一种磁激活细胞分选（MACS）方法，还能富集EpCam阴性的CTCs。因此，我们的方法特别适用于分离具有下调上皮癌变的间充质CTCs，例如易于治疗失败的三阴性乳腺癌患者。

In cancer metastasis, single circulating tumor cells (CTCs) in the blood and disseminated tumor cells (DTCs) in the bone marrow mediate cancer metastasis. Because suitable biomarker proteins are lacking, CTCs and DTCs with mesenchymal attributes are difficult to isolate from the bulk of normal blood cells. To establish a procedure allowing the isolation of such cells, we analyzed the cell line BC-M1 established from DTCs in the bone marrow of a breast cancer patient by stable isotope labeling by amino acids in cell culture (SILAC) and mass spectrometry. We found high levels of the transmembrane protein CUB domain-containing protein 1 (CDCP1) in breast cancer cell lines with mesenchymal attributes. Peripheral blood mononuclear cells were virtually negative for CDCP1. Confirmation in vivo by CellSearch revealed CDCP1-positive CTCs in 8 of 30 analyzed breast cancer patients. Only EpCam-positive CTCs were enriched by CellSearch. Using the extracellular domain of CDCP1, we established a magnetic-activated cell sorting (MACS) approach enabling also the enrichment of EpCam-negative CTCs. Thus, our approach is particularly suited for the isolation of mesenchymal CTCs with downregulated epithelial cancer that occur, for example, in triple-negative breast cancer patients who are prone to therapy failure.