Abstract Ductal carcinoma in situ (DCIS) is an early-stage form of breast cancer that accounts for over 20% of all breast cancers in the US. Therefore, prevention of DCIS is a major goal for reducing breast cancer incidence. However, the underlying molecular and pathological mechanisms of DCIS remain poorly understood. It is believed that tumor-initiating cells are formed at a very early stage of tumorigenesis, which then leads to the progression of DCIS. Our recent work that focused on tumor-initiating cells of DCIS revealed that the expression of miR29c in tumor-initiating cells was significantly down-regulated in DCIS cells compared to that in normal cells. Furthermore, we found that the ID2 gene is one of the targets of miR29c in tumor-initiating cells and that miR29c suppresses the ID2 gene by directly binding to the 3’-UTR sequence. ID2 is a well-known dedifferentiation factor in embryonic development, and the ID family is involved in progression of cancer by affecting cell differentiation and proliferation. We also found that ectopic expression of ID2 in premalignant cells accelerated tumor growth and promoted self-renewal of tumor-initiating cells in DCIS, suggesting that ID2 also controls cancer cell dedifferentiation and promotes tumor-initiating cells formation. These results indicate that the miR29c-ID2 axis plays a critical role in self-renewal of DICS in vivo and that ID2 is a potential biomarker for tumor-initiating cells in DCIS. ID2 is thought to sequester E-protein transcription factors and thus regulates a wide range of gene expression. Our data suggest that ID2 upregulates expression of several cancers stem cell genes including SOX2, which is a well- known self-renewal regulator of stem cells. ID2 also upregulates the fatty acid synthase gene (FASN), which promotes cancer cell proliferation and survival. We also found that FASN expression was significantly suppressed by pterostilbene, indicating potential utility of this natural compound for preventing DCIS formation. Taken together, these results suggest that miR29c-ID2 functions as master regulators in forming DCIS. ID2 promotes self-renewal of tumor-initiating cells by regulating expression of the SOX2 and FASN genes. Note: This abstract was not presented at the meeting. Citation Format: Yin Liu, Puspa R. Pandey, Sambad Sharma, Kerui Wu, Fei Xing, Kounosuke Watabe. Roles of miR29c-ID2 axis in tumor initiating cells of breast ductal carcinoma in situ [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5748. doi:10.1158/1538-7445.AM2017-5748