In the past decades, there has been an increasing evidence for the existence of breast cancer among women worldwide. Despite the progress that has been made in the human breast cancer research, the origin of tumor, maintenance and its resistance to chemotherapy are poorly understood. However, it is already established that breast cancer may be originated and sustained by a small proportion of stem-like self-renewing cells called breast cancer stem cells. Therefore, understanding the role of stem cells in the normal human breast development and its carcinogenesis is crucial in attempts to differentiate the breast cancer stem cells from the bulk of a tumor. This review will focus on the mammary gland development, self-renewal and differentiation of normal breast stem cells, signaling pathways which regulate their selfrenewal and differentiation, deregulation of these normal signaling pathways which might lead to the neoplastic conversion of the normal mammary stem/progenitor cells, how normal mammary stem and their progenitor cells are transformed into breast cancer stem cells, how do they interact with its tissue specific microenvironment, their cell surface markers to identify and target those breast cancer stem cells. Elucidation of these important points is essential to develop novel therapeutic strategies and to improve the current diagnostic techniques.
