for Mammary Stem Cells and Promote Their Long-Te

lo stem cells. MaSCs in vitro because of loss of long-term self-renewal.cancerous cells (Clarke and Fuller, 2006). A critical aspect of the niche model of stem cell regulation is that the availability of self-renewing factors is limited because of their local release. 1994; Weber-Hall et al., 1994). Moreover, it has been suggested that Wnt signals expand mammary stem cells, as indicated by the fact that early tumorigenic lesion from transgenic mice over-Understanding the nature of the signals and their effect on adult stem cells has been complicated because of the low abundance of these stem cells and the paucity of functional assays. More-expressing Wnt1 have increased numbers of stem cells (Nusse and Varmus, 1982; Shackleton et al., 2006; Vaillant et al., 2008). Whether Wnt proteins directly control normal mammaryINTRODUCTION Stem cells are characterized by their ability to self-renew as well as to differentiate into specialized cells, properties critical for tissuemaintenance and regeneration. The decision to self-rene