Viewpoint Highway to heaven: mammary gland development and

In recent years, the mammary gland epithelium has been shown to be a mixture of differentiated cell populations in a hierarchical relationship with their stem and progenitor cells. However, the mechanisms that regulate their cellular differentiation processes are still unclear. The identification of genes that govern stem and progenitor cell expansion, or that determine daughter cell fate, will be of crucial interest for understanding breast cancer diversity and, ultimately, improving treatment. Two recent analyses have identified some of the key genes that regulate these processes, lighting up the highway to normal mammary gland development. In the mammary epithelium, recent successes in identifying stem and progenitor cells and lineage-specific transcription factors have raised the possibility of a much deeper understanding of what controls lineage commitment and cell fate. We have some way to go before we can approach the current level of understanding of haematopoietic lineage commitment. Nevertheless, every new report adds another piece to the puzzle and two recent publications have done just that.