Epithelial differentiation score analysis of normal mammary tissue, human breast tumors, human cell lines and mouse mammary tumors. (a) Differentiation axis based on Lim et al.  data. (b) Mammospheres (MMS; n = 14) derived from normal breast tissue. Yellow crosses identify claudin-low MMS (n = 6, 43%) as defined by the nine-cell line claudin-low predictor. (c) Tumors and the normal breast-like group from the UNC337 database. (d) Breast cancer cell lines. Except for the nine claudin-low cell lines, we used the subtype calls (luminal [L] and basal [B]) as reported in Neve et al. . (e) Mouse tumors from Herschkowitz et al. . (f) Histological special types of breast cancer obtained from the NKI113 database . Colored dots or boxes denote the subtype cells. IDC with OGC, invasive ductal carcinoma with osteoclastic giant cells; ILC, invasive lobular carcinoma; BL (red), basal-like; CL (yellow), claudin-low defined by the nine-cell line claudin-low predictor; H2 (pink), HER2-enriched; LA (dark blue), luminal A; LB (light blue), luminal B; NBL (green), normal breast-like. *P < 0.0001.