Understanding the origins of hematopoietic stem cells has been a challenge due to the lack of a marker specific to this cell type. Recently, our lab has functionally proven that HoxB5 is a unique marker for long-term HSCs in adult murine models. Whether HoxB5 is also a marker for HSCs in development, has not been studied. Here, we are labeling HoxB5+ cells found in the yolk sac blood islands at E7.5 by using a genetic tool we generated. We will then analyze all marked cells at several time points during development and in adulthood. This project will discover if HoxB5+ cells from the yolk sac contribute to the adult hematopoietic system and the LT-HSC pool.