The present invention relates to organic molecules capable of modulating tyrosine kinase signal transduction and particularly KDR/FLK-1 receptor signal transduction in order to regulate and/or modulate vasculogenesis and angiogenesis. The invention is based, in part, on the demonstration that KDR/FLK-1 tyrosine kinase receptor expression is associated with endothelial cells and the identification of vascular endothelial growth factor (VEGF) as the high affinity ligand of FLK-1. These results indicate a major role for KDR/FLK-1 in the signaling system during vasculogenesis and angiogenesis. Engineering of host cells that express FLK-1 and the uses of expressed FLK-1 to evaluate and screen for drugs and analogs of VEGF involved in FLK-1 modulation by either agonist or antagonist activities is also described. The invention also relates to the use of the disclosed compounds in the treatment of disorders, including cancer, diabetes, hemangioma and Kaposi's sarcoma, which are related to vasculogenesis and angiogenesis.