Transcriptional regulatory networks pay a fundamental role in determination of stem cell fate. However, relatively little is known about the regulatory transcription factors involved in mammary stem cell (MaSC) self-renewal and differentiation. Our lab and others have shown that Id4 marks a stem/progenitor cell in the MaSC-enriched basal compartment of the mammary epithelium where it acts as a master negative regulator of mammary gland development by supressing key pathways critical for luminal fate commitment; namely Notch, BRCA1, Elf5 and the Estrogen Receptor^1,2. Our lab has also demonstrated that Id4 is expressed by a subset of aggressive basal-like breast cancers (BLBC) where it is required for proliferation. Using a novel reporter mouse in which EGFP is knocked into the Id4 locus has allowed us to prospectively isolate Id4-positive mammary cells and investigate their function in vitro and in vivo. When Id4-positive basal cells are transplanted into cleared mammary fat pads of recipient mice they are able to form ductal outgrowths more efficiently than Id4-negative basal cells, demonstrating that these cells possess self-renewal and multi-lineage differentiation capacity – both hallmarks of stem cells. Id transcription factors have pivotal roles in stem cell maintenance and are traditionally thought to function by negatively regulating bHLH transcription factors. We propose a new model for Id4 function whereby Id4 acts at chromatin to modify transcriptional complexes. We are using a new technique called Rapid Immunoprecipitation Mass Spectrometry of Endogenous Proteins (RIME) to investigate Id4 protein binding partners in the normal murine cell line, Comma Dβ. We are also investigating putative DNA binding sites and gene targets of Id4 by ChIP-Seq and RNA-Seq. It is essential to understand the way in which this master regulator functions in normal development to understand its role in BLBC, and how we may be able to subsequently intervene in tumour progression.