Transcription factors are essential for controlling cell fate decisions and differentiation during embryonic and adult development. Although transcription factors have been implicated in regulating development of the mammary gland, most of these have not been defined at a cellular level. Recent studies in the laboratory have identified the transcription factor Runx2 as a highly expressed gene in the mammary stem cell (MaSC) enriched population of both mouse and human mammary glands. Runx2 is a member of the RUNX family, which is characterized by the presence of a Runt DNA-binding domain, and has been shown to be essential for osteogenesis. Runx2 expression in mammary epithelial cells has revealed a potential role in the metastasis of mammary tumours to bone, and in high-grade breast cancers, Runx2 expression is positively correlated with ER expression. Runx2 has been shown to activate the expression of Osteopontin in the mammary gland, a secreted glycophosphoprotein protein, which plays an essential role in mammary gland differentiation. When Osteopontin is inhibited, luminal cells are unable to differentiate during pregnancy into milk producing cells resulting in lactation deficiency. We have developed conditional knockout mouse models to investigate the function of Runx2 in mammary gland development. By using these mouse models, we have shown that Runx2 deficiency has no impact on development of the virgin mammary gland. During pregnancy, Runx2 deficiency causes a differentiation block that results in the accumulation of alveolar progenitors and a subsequent decrease in mature epithelial cells, leading to a decrease in epithelium of the pregnant gland.