Cell-cell communication during vascular development and tumour angiogenesis seems to involve at least five endothelial cell-specific tyrosine kinase receptors belonging to two distinct subclasses: two receptors of the Tie family, and three vascular endothelial cell growth factor receptors, VEGFR-1, -2 and -3, originally named Fltl (Fms-like tyrosine kinase), KDR/Flk-1 (Kinase insert-domain containing receptor or fetal-liver kinase-1) and Flt4, respectively. VEGFRs are subclass-III receptor tyrosine kinases, homologous to the platelet-derived growth factor (PDGF)-receptor family, having seven immunoglobulin homology domains in the extracellular domain, and a tyrosine kinase intracellular domain split by a kinase insert sequence (for recent reviews, see Klagsbrun and D’Amore 1996; Folkman and D’Amore 1996; Mustonen and Alitalo 1995; Korpelainen and Alitalo, 1998, Claesson-WELSH, this book).