MUC1 and breast cancer: In the case of patient #8 where epithelial-mesenchymal phenotype was evaluable both in tumor biopsy and CTCs, the trend towards a more mesenchymal phenotype at progression was echoed in the CTC analysis as the mesenchymal (Muc1/CEA+, vimentin+, CK−) and transitional (Muc1/CEA+, vimentin+, CK+) subpopulations of breast cancer CTCs became the dominant phenotypes at progression over the epithelial subpopulation of CTCs (Muc1/CEA+, vimentin−, CK+) that was the dominant phenotype at time of enrollment.