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 transcriptional regulator Inhibitor of Differentiation 4 (Id4) as a highly expressed gene in the mammary stem cell (MaSC)-enriched population of both mouse and human mammary glands. This population also contains progenitors and mature myoepithelial cells. Germ-line deletion of Id4 in mice has previously revealed a role for this gene in neuronal differentiation1 , and more recently, as a regulator of mammary gland development through its suppression of p38MAPK2 . We have developed a conditional knockout mouse model to investigate the function of Id4 specifically in the mammary epithelium. Id4 is expressed in myoepithelial cells of the terminal end buds and ducts, with expression reducing after puberty. Consistent with previous findings2 , Id4-deficient mammary glands displayed a prominent reduction in ductal elongation and branching. We found that Id4-deficient mammary epithelial cells also have a diminished capacity to maintain the balance of luminal to myoepithelial cells, and that the MaSC/basal subset had significantly reduced repopulating capacity when assayed by transplantation. Understanding the mechanism and role of Id4 in the developing mammary gland may reveal important parallels with the role of Id4 in breast cancer.