A highly tumourigenic subpopulation of cancer cells has recently been identified in primary and metastastatic breast cancer. This minority of cells termed tumour-initiating cells or “cancer stem cells“ (CSC) has been shown in several solid tumours to bear stem cell features such as the capacity to self-renew and to cause the heterogeneous lineages of cancer cells that comprise the tumour. Correlating with this, several pathways and genes required for normal stem-cell function are activated in cancer cells and play essential roles in tumourigenesis. However, the specific molecular mechanisms through which CSCs exert their tumourigenic effects remain unknown. The TGF-beta pathway is a key player in mammalian biology, and its misregulation can result in tumour development. Increasing evidences are starting to elucidate the mechanistic basis and clinical relevance of TGF-beta’s role in cancer, yet its complex and pleiotropic nature requires still a much profound understanding of the pathway. The proposed project aims to study a yet unidentified role of TGF-beta in breast cancer stem cell regulation by analyzing human breast cancer cell lines as well as fresh breast tissue samples derived from primary tumours and malignant pleural effusions from patients with breast cancer of diverse grade and stage. TGF-beta regulation of the different subtypes of breast progenitor cells will be assess by colony forming cell and mammosphere-initiating cells assays as well as by an innovative xenotransplantation assay in vitro and in vivo respectively. Moreover, a transcriptomic approach will be designed to discern the mechanisms and molecular determinants of TGF-beta's role in breast stem cell regulation. By developing this project, we will decipher how TGF-beta regulates breast stem/progenitor cell functions, paving the way for a better understanding of its complexity and therapeutic potential.