Several factors appear to play a role in the engraftment of hematopoietic stem cells (HSCs) in the bone marrow (BM). HSC (blue nuclei and cytoplasm) must gain access to the bone marrow through the vasculature (red) in a process termed homing, take up residence in the BM, and then undergo self-renewing cell divisions to produce a larger pool of HSC, and differentiate to generate more committed progenitors (other cell types indicated). Although cytokines and growth factors that play a role in HSC differentiation are well known, less is known of cytokines and signals that support HSC expansion. Along with the better studied c-Kit–ligand (KL) and, possibly, the ligand for the receptor Flt3 (FL), thrombopoietin (TPO) is now proposed to play an important role in HSC expansion in the engraftment process.