Abstract Background CD133 and CD117 represent markers of human breast tissue differentiation. CD133 is an early progenitor marker while CD117 represents late luminal progenitor marker (Lim, et al, Nature Medicine 2009). Morphologic expression of the two markers has not been well established in paraffin embedded tissue of human breast lesions. Design In this study, we evaluated CD133 and C117 expression in different human breast lesions. Group 1 consisted of 15 benign breast cases (5 reduction mammoplasty cases with no significant pathology and 10 fibroadenoma or fibrocystic changes). Group 2 (malignant group 2) included 13 breast cases with invasive ductal carcinoma (IDC). Group 3 (malignant group 3) was composed of 12 BRCA-mutant breast cases but only six cases had IDC. All sections from paraffin embedded tissue were immunohistochemically stained for CD133 (AC133 clone) and CD117 (monoclonal antibody). Expression of the two markers in normal breast glands adjacent to lesions (normal controls), benign lesions (benign controls) and IDC were evaluated and positive rate was calculated. Results Both CD133 and CD117 were expressed in normal breast glands (normal controls) of all three groups and benign breast lesions (Table 1). However, the expression of CD133 was detected in the majority of IDC lesions as compared to low positive percent of CD117 expression in both groups 2 and 3 (Table 1). Recombined tumor cases, based on the molecular luminal status, also showed 56 - 70% CD133 positivity but 0 - 20% CD117 positivity in IDC cells. Table 1. CD133 and CD117 in benign and malignant breast lesions Group 1, total Group 2, total Group 3, total CD133+ in normal 15/15 (100%) 13/13 (100%) 12/12 (100%) CD117+ in normal 15/15 (100%) 13/13 (100%) 11/12 (91%) Group 1, Benign Lesions Group 2, Invasive cancer Group 3 Invasive cancer CD133+ in lesions 10/10 (100%) 8/13 (61%) 4/6 (67%) CD117+ in lesions 9/10 (90%) 1/13 (8%) 1/6 (17%) Conclusions We show that normal breast glands expressed both CD133 and CD117, compatible with the presence of stem cell niche in normal breast ducts and acini. However our data suggest that in IDC, there is presence of early progenitor cells but not late luminal progenitor cells, regardless of the molecular luminal status, supporting the view that cancer stem cells exist in the invasive breast carcinoma. Citation Format: Kevin J. Zhang, Suling Liu, Nava Siegelmann-Danieli, Ping L. Zhang. Early progenitor cells but not late luminal progenitor cells preserve in invasive breast carcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5022. doi:10.1158/1538-7445.AM2013-5022