The isolation of mouse MaSCs capable of forming a functional mammary gland from a single cell has opened up many new avenues of research to study the function and regulation of these cells. These avenues will diverge down many paths, from studying the role of MaSCs in mammary gland development, to understanding the regulation of self renewal and differentiation of MaSCs, to investigating a potential role of MaSC in breast tumorigenesis. Recent studies have begun to unveil the complex nature of MaSC regulation. For example, CD29 (β1 integrin) and the Wnt receptor Lrp5 are found to be important for MaSC function 
, while the differentiation of MaSCs has been shown to be influenced by the transcription factor GATA-3 
and Notch signaling 
. Furthermore, the growth of MaSCs is likely to be directly regulated by epidermal growth factor via binding to EGFR/ErbB1 but only indirectly by estrogen or progesterone since they do not express their respective receptors 
. Despite these recent advances, better methods to sensitively and non-invasively monitor MaSC activity in vivo
are urgently needed to facilitate future study of MaSC function and regulation. The model we have presented in this study will be a valuable research platform in the characterization of MaSCs since it is the only tool available for quantitative real-time tracking of MaSC-enriched cells in living animals. Few if any other adult stem cell models allow for both single marker histological analysis of adult stem cell localization and
the ability to monitor adult stem cell activity in individual mice over time. Our model represents a significantly simplified surrogate to the more cumbersome FACS methods which can not be used to assess MaSC activity longitudinally in living animals and requires a large number of animals to obtain data at different time points of the experiment. Using this model in combination with immunohistochemistry, we have identified the location of luciferase expressing MaSCs in both a luminal and basal compartment. Future characterization will help to elucidate whether these two epithelial populations are functionally distinct from one another. Furthermore, this model can be coupled with genetic manipulations of MaSCs in vivo
or ex vivo 
prior to transplantation or with manipulation of the host microenvironment to study how MaSCs are regulated by intrinsic signaling pathways or extrinsic cues from their surrounding niche.
We have also used this model to monitor MaSC dynamics throughout pregnancy, where we have shown that MaSCs rise in both total number and percentage during pregnancy and then decline to or below baseline levels after weaning. This effect is dampened when mothers do not nurse their young. Additionally, the magnitude of MaSC expansion is decreased during a second pregnancy. These results are particularly interesting when considered in the context of breast cancer, as it is known that in both mice and humans, full-term and multiple pregnancy results in a short term increase, but ultimately a long term decrease in breast cancer susceptibility 
. It has been proposed that this is due to the transient expansion of MaSCs during pregnancy and their depletion afterwards, as MaSCs or progenitor cells may serve as the particularly susceptible cellular targets for transformation. Curiously, this protective effect is weakened when mothers do not nurse their young. Future studies should directly test the susceptibility of MaSCs to transformation to better understand how changes in their cell number throughout pregnancy may alter the likelihood of developing breast cancer.
In summary, our model serves as a unique platform in which MaSC activity can be non-invasively, sensitively and quantitatively monitored by bioluminescence imaging. This system will not only allow for the monitoring of MaSC activity in normal physiologically relevant processes but will also permit more direct evaluation of the susceptibility of MaSCs to oncogenic transformation and the regulation of their growth throughout tumorigenesis. If MaSCs play a critical role in the initiation or progression of breast cancer, this model will serve as an ideal system to develop and test future therapeutic applications that target MaSCs, including potentially novel prophylactic breast cancer treatments for high risk groups.