Since the recognition of basicfibroblast growth factor (bFGF, FGF-2) and acidic FGF (aFGF, FGF-1) as proliferation and maintenance factors for endothelial cells in culture,1,2 much interest has arisen regarding their role as potential angiogenic factors in tumors and tissue healing in vivo. The FGFs, of which presently at least 22 members have been recognized,3 interact with FGF receptors (FGFRs). After FGF binding, the FGFRs dimerize and become activated displaying protein tyrosine kinase activity and subsequent signaling in the cell.4,5 Four functional FGFRs have been recognized, FGFR1–4, most of which can be encountered in various alternative spliced isoforms. FGFR1 plays an essential role during early stages of development of the mesoderm and at several levels during organogenesis.3,6 FGF/FGFR1 is expressed almost exclusively in the mesenchyme, including endothelial cells, smooth muscle cells, and hemopoietic progenitor cells. FGFR1 is involved in the stimulation and proliferation of endothelial cells and vascularization by FGFs, particularly bFGF and aFGF. FGFs also promote the formation of hematopoietic progenitor cells in early development, which requires the FGFR1 as well.7,8 In line with this, chromosomal translocation involving FGFR1 has been observed in patients with stem-cell myeloproliferative disease, probably caused by constitutive activation of the FGFR1 gene.9 See page 944 It has long been anticipated that hematopoietic progenitor cells and endothelial progenitor cells derive from a common cell type, the hemangioblast.10 Studies on mouse embryos have shown that such hemangioblast cells indeed exist. These cells develop in the primitive streak of the embryo and migrate to the yolk sac, where they form blood islands.11 The outer cells of these blood islands mature into endothelial cells and form a vascular plexus, while the inner cells develop into primitive and definitive hematopoietic progenitor cells and their subsequent lineages of blood cells. In addition, in …