Surface markers EpCAM, CD24, and CD49f classify breast cancer cell lines into distinct differentiation states. (a) Cell surface markers EpCAM, CD24, and CD49f distinguish four classes of cell lines that map to differentiation states found in normal precursor cells. Surface marker expression was quantified by flow cytometry (mean ± S.D., n = 3 to 4 biological replicates per cell line) as described in Materials and Methods. (b) Luminal 1, Luminal 2, Basal, and Mesenchymal cell lines identified by EpCAM, CD24, and CD49f expression were classified on the basis of CK14, CK8/18, ERα, EpCAM, and vimentin expression. Representative immunofluorescence images are shown from MCF7 (Luminal 1), BT20 (Luminal 2), HCC1806 (Basal), and MDA.MB.231 (Mesenchymal). Nuclei were counterstained with DAPI (blue). ER expression of Luminal II cells is heterogeneous across cell lines. Cell lines T47 D, HCC1500, and MDA.MB.361 all express ER (MDA.MB.361 is shown in inset), while BT20, SUM225 and HCC1419 are ER-negative. Original magnification: 200×. Bar = 100 μm.