Purpose: To evaluate the expression of stem cell–related markers at the cellular level in human breast tumors of different subtypes and histologic stage.

Experimental Design: We performed immunohistochemical analyses of 12 proteins [CD44, CD24, ALDH1, vimentin, osteonectin, EPCR, caveolin 1, connexin 43, cytokeratin 18 (CK18), MUC1, claudin 7, and GATA3] selected based on their differential expression in breast cancer cells with more differentiated and stem cell–like characteristics in 47 cases of invasive ductal carcinoma (IDC) only, 135 cases of IDC with ductal carcinoma in situ (DCIS), 35 cases of DCIS with microinvasion, and 58 cases of pure DCIS. We also analyzed 73 IDCs with adjacent DCIS to determine the differences in the expression of markers by histology within individual tumors. CD44+/CD24− and CD24−/CD24+ cells were detected using double immunohistochemistry.

Results: CD44 and EPCR expression was different among the four histologic groups and was lower in invasive compared with in situ tumors, especially in luminal A subtype. The expression of vimentin, osteonectin, connexin 43, ALDH1, CK18, GATA3, and MUC1 differed by tumor subtype in some histologic groups. ALDH1-positive cells were more frequent in basal-like and HER2+ than in luminal tumors. CD44+/CD24− cells were detected in 69% of all tumors with 100% of the basal-like and 52% of HER2+ tumors having some of these cells.

Conclusions: Our findings suggest that in breast cancer, the frequency of tumor cells positive for stem cell–like and more differentiated cell markers varies according to tumor subtype and histologic stage. Clin Cancer Res; 16(3); 876–87